Determining a resolution state of an anchor constant associated with an application programming interface (api) point

ABSTRACT

A parametric constant resolves to different values in different contexts, but a single value within a particular context. An anchor constant is a parametric constant that allows for a degree of parametricity for an API point. The context for the anchor constant is provided by a caller to the API point. The anchor constant resolves to an anchor value that records specialization decisions for the API point within the provided context. Specialization decisions may include type restrictions, memory layout, and/or memory size. The anchor value together with an unspecialized type of the API point result in a specialized type of the API point. A class object representing the specialized type is created. The class object may be accessible to the caller, but the full value of the anchor value is not accessible to the caller. The API point is executed based on the specialization decisions embodied in the anchor value.

RELATED APPLICATIONS; INCORPORATION BY REFERENCE

This application hereby incorporates by reference the followingapplications: Application No. 63/163,697, filed on Mar. 19, 2021, titled“Dynamically Imposed Field and Method Type Restrictions For ManagedExecution Environments”; Application No. 63/163,701, filed on Mar, 19,2021, titled “Optional Specialization of Generic Types and Methods”;Application No. 63/163,723, filed on Mar. 19, 2021, titled “EncapsulatedSpecialization of Dynamically-Linked API Points”; Application No.63/163,726, filed on Mar. 19, 2021, titled “Pointwise and ReplicativeType Specializations”; Application No. [R00638NP], filed on Jan. 7, 2022titled “Dynamically-Imposed Field and Method Type Restrictions forManaged Execution Environments”; Application No. [R00639NP], filed onJan. 7, 2022 titled “Implementing Optional Specialization When ExecutingCode”; Application No. [R00639N2], filed on Jan. 7, 2022 titled“Implementing Optional Specialization When Compiling Code”; ApplicationNo. [R00640NP], filed on Jan. 7, 2022 titled “Determining differentresolution states for a parametric constant in different contexts”;Application No. [R00640N3], filed on Jan. 7, 2022 titled “Executing aparametric method within a specialized context”; Application No.[R00640N4], filed on Jan. 7, 2022 titled “Instantiating a parametricclass within a specialized context”; Application No. [R00640N5], filedon Jan. 7, 2022 titled “Accessing a parametric field within aspecialized context”; Application No. [R00641NP], filed on Jan. 7, 2022titled “Implementing a type restriction that restricts to anon-polymorphic layout type or a maximum value”; Application No.[R00641N2], filed on Jan. 7, 2022 titled “Implementing a typerestriction that restricts to a singleton value or zero values” andApplication No. [R00641N3], filed on Jan. 7, 2022 titled “Implementing atype restriction that restricts to a maximum or specific element count”;each of which is hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to parametric constants and applicationprogramming interface (API) points. In particular, the presentdisclosure relates to determining different resolution states for aparametric constant in different contexts.

BACKGROUND

Compilers for different programming languages may perform differenttypes of compilations. In some instances, a compiler converts sourcecode, which is written according to a specification directed to theconvenience of the programmer, to machine or object code. Machine orobject code is executable directly by the particular machineenvironment. In other instances, a compiler converts source code to anintermediate representation (also known as “virtual machine code” or“virtual machine instructions”), such as bytecode, which is executableby a virtual machine that is capable of running on top of a variety ofparticular machine environments. The virtual machine instructions areexecutable by the virtual machine in a more direct and efficient mannerthan the source code. Converting source code to virtual machineinstructions includes mapping source code functionality from thelanguage to virtual machine functionality that utilizes underlyingresources, such as data structures. Often, functionality that ispresented in simple terms via source code by the programmer is convertedinto more complex steps that map more directly to the instruction setsupported by the underlying hardware on which the virtual machineresides.

A set of code includes a set of application programming interface (API)points. An API point is a named class, interface, method, constructor,or field. Generics enable types (classes and interfaces) to beparameters when defining API points. A parametric API point is an APIpoint that is parameterized over types.

The approaches described in this section are approaches that could bepursued, but not necessarily approaches that have been previouslyconceived or pursued. Therefore, unless otherwise indicated, it shouldnot be assumed that any of the approaches described in this sectionqualify as prior art merely by virtue of their inclusion in thissection.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments are illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings. It should benoted that references to “an” or “one” embodiment in this disclosure arenot necessarily to the same embodiment, and they mean at least one. Inthe drawings:

FIG. 1 illustrates an example computing architecture in which techniquesdescribed herein may be practiced.

FIG. 2 is a block diagram illustrating one embodiment of a computersystem suitable for implementing methods and features described herein.

FIG. 3 illustrates an example virtual machine memory layout in blockdiagram form according to an embodiment.

FIG. 4 illustrates an example frame in block diagram form according toan embodiment.

FIGS. 5A-5B illustrate state machines for constants, and in particular,parametric constants, according to an embodiment.

FIG. 6 illustrates runtime data areas storing information for constants,including parametric constants, according to an embodiment.

FIG. 7 illustrates a call stack storing information for constants,including parametric constants, according to an embodiment.

FIG. 8 illustrates a class file including one or more parametric APIpoints according to an embodiment.

FIG. 9 illustrates information structures for a specialization anchorconstant and a specialization linkage constant according to anembodiment.

FIGS. 10A-10B illustrate state machines for a specialization anchorconstant and a specialization linkage constant according to anembodiment.

FIG. 11 illustrates storage of an anchor value included in resolvedvalues for resolution states of a specialization anchor constant and aspecialization linkage constant according to an embodiment.

FIG. 12A illustrates an unspecialized type and a specialized type thatis the unspecialized type specialized within a particular contextaccording to an embodiment.

FIG. 12B illustrates an object of a specialized type according to anembodiment.

FIG. 13 illustrates a resolved value, namely an anchor value, for aresolution state of a specialization anchor constant according to anembodiment.

FIG. 14 illustrates encapsulation of specialization decisions viarestrictions on loadable values of specialization linkage constantsaccording to an embodiment.

FIG. 15 illustrates a call to a parametric API point according to anembodiment.

FIG. 16 illustrates an example set of operations for preparing andresolving a type in a runtime environment, including preparing andresolving a parametric constant within a default context, according toan embodiment.

FIG. 17 illustrates an example set of operations for executing aparametric API point within a particular context, including selecting aresolution state of a parametric constant associated with the particularcontext, according to an embodiment.

FIG. 18 illustrates an example set of operations for preparing andresolving a parametric constant within particular contexts provided bycalls to parametric API points dependent on the parametric constantaccording to an embodiment.

FIG. 19 illustrates an example set of operations for determining ananchor value for a resolution state of an anchor constant according toan embodiment.

FIG. 20 illustrates an example set of operations for executing aninvocation instruction to invoke a parametric method according to anembodiment.

FIG. 21 illustrates an example set of operations for executing acreation instruction to create an instance of a parametric typeaccording to an embodiment.

FIG. 22 illustrates an example set of operations for executing an accessinstruction to access a parametric field according to an embodiment.

FIG. 23 illustrates a system in accordance with one or more embodiments.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding. One or more embodiments may be practiced without thesespecific details. Features described in one embodiment may be combinedwith features described in a different embodiment. In some examples,well-known structures and devices are described with reference to ablock diagram form in order to avoid unnecessarily obscuring the presentinvention.

-   -   1. GENERAL OVERVIEW    -   2. ARCHITECTURAL OVERVIEW    -   2.1 EXAMPLE CLASS FILE STRUCTURE    -   2.2 EXAMPLE VIRTUAL MACHINE ARCHITECTURE    -   2.3 LOADING, LINKING, AND INITIALIZING    -   3. PARAMETRIC CONSTANTS    -   4. PARAMETRIC APPLICATION PROGRAMMING INTERFACE (API) POINTS    -   5. RESOLVING A SPECIALIZATION ANCHOR CONSTANT AND A        SPECIALIZATION LINKAGE CONSTANT    -   6. EXECUTING PARAMETRIC API POINTS WITHIN PARTICULAR CONTEXTS    -   7. EXAMPLE EMBODIMENTS    -   7.1 EXECUTING A PARAMETRIC METHOD DEPENDENT ON AN ANCHOR        CONSTANT    -   7.2 CREATING AN INSTANCE OF A PARAMETRIC CLASS DEPENDENT ON AN        ANCHOR CONSTANT    -   7.3 ACCESSING A PARAMETRIC FIELD DEPENDENT ON AN ANCHOR CONSTANT    -   8. MISCELLANEOUS; EXTENSIONS    -   9. HARDWARE OVERVIEW

1. General Overview

One or more embodiments include determining different resolution statesfor a parametric constant in different contexts. An invariant constanthas at most one value. A state machine for an invariant constantincludes an initial unresolved state and then a resolution state, and noother states. When an invariant constant is prepared, the invariantconstant assumes the unresolved state. When an invariant constant isresolved, the invariant constant transitions to the resolution state.The value (whether a resolved value or an error) determined for theresolution state is not changeable. An invariant constant can beprepared at most once; hence the invariant constant can assume at mostone value. In contrast, a parametric constant can be associated withmultiple values, each value being the parametric constant's sole valuewithin a respective context. A state machine for a parametric constantis similar to that for an invariant constant—the state machine includestwo states, an unresolved state and a resolution state. The valuedetermined for each resolution state is not changeable. However, aparametric constant be prepared multiple times. Each preparationcorresponds to at most a single resolution. Each unresolved statetransitions into at most a single resolution state. Different resolutionstates are associated with different contexts; however the valuedetermined for each resolution state is not changeable and thereforeconstant for each context. Further, a constant (whether invariant orparametric) is associated with at most one bootstrap method. A bootstrapmethod determines a resolved value for a resolution state of a constant.For a parametric constant, the same bootstrap method is executed fordifferent resolutions, but each execution receives different contexts asinput.

A context for a parametric constant is information relevant todetermining a resolved value for the parametric constant that issupplied by a user of the parametric constant. A user of the parametricconstant may be, for example, a caller of an application programminginterface (API) point that depends on the parametric constant. Thecontext is applicable and unchangeable during the execution of the APIpoint. Hence the context is applicable and unchangeable while within astack frame, on a call stack, corresponding to the execution of the APIpoint. As described above, each context corresponds to at most oneresolution state, and one resolved value, of a parametric constant.Therefore all operations within the stack frame corresponding to theexecution of the API point can access at most one resolution state, andone resolved value, of the parametric constant. Within the stack framecorresponding to the execution of the API point, the parametric constanthas a single constant value. Indeed, within any stack framecorresponding to execution of any API point depending on the sameparametric constant, wherein each respective caller supplies the samecontext, the parametric constant has a single constant value. However,within different stack frame corresponding to executions of the same APIpoint within different contexts, operations of different stack framesaccess different resolution states, and different resolved values, ofthe same parametric constant.

A per-type runtime constant pool stores a set of constants for a type. Aper-type runtime constant pool includes a single invariant runtimeconstant pool, storing invariant constants, and zero or more parametricruntime constant pool, storing parametric constants. For a particularparametric constant, at most a single resolution state is stored in eachparametric runtime constant pool. A parametric runtime constant poolstoring a resolution state of a parametric constant relevant to one ormore contexts is also associated with the same one or more contexts.(While a context is associated with at most one resolution state of aparticular parametric constant, a resolution state may be associatedwith one or more contexts.) Therefore, different parametric runtimeconstant pools associated with the same parametric constant areassociated with different contexts. As used herein, the term “runtimeconstant pool” may refer to a per-type runtime constant pool, aninvariant runtime constant pool, and/or a parametric runtime constantpool.

Each stack frame is associated with a constant pool reference, pointingto a per-type runtime constant pool for a type defining (or inheriting)the API point being executed. Operations within a stack frame accessresolution states of constants from the per-type runtime constant poolthat is referenced by the stack frame's constant pool reference. Aconstant pool reference includes one component pointing to the invariantruntime constant pool for a particular type. Additionally the constantpool reference includes another component pointing to a parametricruntime constant pool associated with the particular type. Whilepotentially multiple parametric runtime constant pools are associatedwith the particular type, the referenced parametric runtime constantpool corresponds to the same context that is applicable to the stackframe. Since a stack frame is associated with at most one of a set ofcontexts associated with a particular parametric constant, and a contextis associated with at most one parametric runtime constant poolassociated with the particular parametric constant, therefore at mostone parametric runtime constant pool associated with the particularparametric constant is accessed for the duration of each stack frame; noother parametric runtime constant pool associated with the particularparametric constant is accessed.

One or more embodiments include determining a resolution state of aspecialization anchor constant (also referred to as an “anchorconstant”) on which an API point depends. An anchor constant is aparametric constant. An API point that depends on a parametric constantmay be referred to as a “parametric API point.” A resolved value for aresolution state of an anchor constant stores a set of specializationdecisions for the parametric API point within a particular context. Aresolved value for a resolution state of an anchor constant may bereferred to herein as a “specialization anchor value,” or “anchorvalue.” An anchor value may be stored in the form of an object of aparticular type, referred to herein as a “specialization anchor type,”or “anchor type.”

An “unspecialized type” is a type that defines one or more API points.An unspecialized type may specify that a particular API point (definedby the unspecialized type itself, or defined by a supertype of theunspecialized type and inherited by the unspecialized type) is dependenton an anchor constant, thereby rendering the particular API pointparametric. The unspecialized type thereby also depends on the anchorconstant and is parametric.

A “specialized type” is a type resulting from specializing a parametricunspecialized type within a particular context. The specialized type isthe parametric unspecialized type adorned with specialization decisions(which may be embodied in an anchor value). The parametric unspecializedtype may be referred to as an “underlying unspecialized type” for thespecialized type. Although the specialized type itself does not defineor declare any fields and methods, the specialized type has the samefields and methods as the underlying unspecialized type. The fields andmethods of the specialized type are the same as the fields and methodsof the underlying unspecialized type, but the fields and methods of thespecialized type are further adorned with specialization decisions.

Specialization decisions may include type restrictions on fields, methodparameters, and/or method return values. A type restriction on an APIpoint in a specialized type prohibits the API point from assuming one ormore values that are originally valid for a type of the same API pointas specified by the underlying unspecialized type.

Additionally or alternatively, specialization decisions may include adifferent memory layout and/or memory size for a specialized typerelative to the underlying unspecialized type. As an example, anunspecialized type may specify that a field is of the type Object. Hencea memory space for the field stores a reference value, which may require8 bytes. A specialized type may include a type restriction on the field,restricting the field to values of int. Values of int may be storedinline and require only 4 bytes. Hence, a memory layout associated withthe specialized type may include inlining the int value for the fieldwithin an instance of the specialized type. Further, a memory sizeassociated with the specialized type may be 4 bytes less than a memorysize associated with the unspecialized type.

Based on specialization decisions, further customizations and/oroptimizations may be performed. Memory flattening optimizations refer tostoring a value in place of a reference to the value, also referred toas “inlining.” The value may be associated with a field, methodparameter, and/or method return value. The inlining may occur in localvariables in a call frame, an operand stack, and/or the passing ofvalues between a callee method and a caller method. Type sharpeningoptimizations refer to eliding certain operations of a method. As anexample, a method may specify that different operations are performedbased on a type of a method parameter. For example,

void m(T t) {  if (t instanceof String) { a(t); }  else b (t); }When specializing to T=String, the type test and the else-arm areelided; when specializing to T!=String, the type test and the if-arm areelided.

As described above, an anchor value stores a set of specializationdecisions, within a particular context, for an API point that depends onan anchor constant. The particular context is associated with aspecialization proposal supplied by the caller of the API point. Theproposal is specified by the caller using an operand of an instructionthat calls the API point. The operand is a specialization linkageconstant (also referred to as a “linkage constant”). The linkageconstant is a wrapper around the API point and a specialization selectorvalue (also referred to as a “selector value”). The selector valueindicates the caller's specialization proposal for the API point.

A linkage constant is initially in an unresolved state. The linkageconstant wraps around a parametric API point (specifically, a symbolicreference to the parametric API point) and a selector value. If theselector value is unvalidated, resolving the linkage constant includesresolving an anchor constant on which the parametric API point depends.Resolving the anchor constant includes verifying whether the proposedselector value is valid for the anchor constant. The callee may acceptor reject the selector value. If the selector value is rejected, anerror is permanently stored as the resolution state of the anchorconstant within the context associated with the selector value. If theselector value is accepted, then the callee determines an anchor valueembodying specialization decisions made based on the selector value. Theabove validation and resolution operations may be embodied in abootstrap method corresponding to the anchor constant. The anchor valueis stored as a resolved value for a resolution state of the anchorconstant. Then both the parametric API point (specifically, a directreference to the parametric API point) and the anchor value are storedas a resolved value for a resolution state of the linkage constant. Theparametric API point is then executed based on the anchor value.

Alternatively, a linkage constant in an unresolved state may wrap arounda validated selector value. A selector value is validated if theselector value matches an anchor value previously resolved for theanchor constant on which the parametric API point depends. Theparametric API point is then executed based on the matching anchorvalue.

As described above, a resolved linkage constant wraps around aparametric API point and an anchor value. The resolved linkage constantmay be used in a subsequent call to any parametric API point dependenton the same anchor constant. The called parametric API point is thenexecuted based on the anchor value indicated by the resolved linkageconstant.

One or more embodiments include executing a parametric method within aparticular context. An invocation instruction to invoke a parametricmethod supplies a context. Information indicating the context (such asan anchor value) is passed from a stack frame associated with the callerto a stack frame associated with the callee. Hence, the contextapplicable to execution of the parametric method is accessible tooperations within the stack frame associated with the callee.

In an embodiment, a context is associated with a type restriction on amethod parameter and/or method return value. Hence, during execution ofthe parametric method, the method parameter and/or method return valueis processed based on the type restriction. Processing a methodparameter and/or method return value based on a type restriction mayinclude type checking the method parameter and/or method return valuebased on the type restriction, and/or converting or casting the methodparameter and/or method return value based on the type restriction.Additionally or alternatively, processing a method parameter and/ormethod return value based on a type restriction may include performingcertain optimizations including memory flattening and/or stackflattening, such as modifying a memory size for the method parameterand/or method return value based on the type restriction, and/orinlining the method parameter and/or method return value based on thetype restriction.

In an embodiment, a context is associated with one of a plurality ofparametric runtime constant pools associated with an anchor constant onwhich a parametric method depends. Each of the plurality of parametricruntime constant pools stores a respective resolution state for theanchor constant. Hence, a constant pool reference of a stack framecorresponding to execution of the parametric method within a particularcontext points to the parametric runtime constant pool associated withthe same particular context. Operations within the stack frame access aresolution state for the anchor constant, and resolution states for zeroor more other constants dependent on the anchor constant, from thereferenced parametric runtime constant pool; no other parametric runtimeconstant pool associated with the anchor constant is accessed within thestack frame. The parametric method may be executed using the resolutionstates within the referenced parametric runtime constant pool, withoutusing resolution states within any other parametric runtime constantpool associated with the anchor constant.

One or more embodiments include creating an object of a specializedtype. Generating an object involves the steps of allocating memory spaceto the object and initializing the object. Generating an object of aspecialized type involves (a) allocating memory space based on a memorysize and/or memory layout associated with the specialized type, and (b)initializing the object based on the underlying unspecialized type.

In an embodiment, different instructions perform memory allocation andinitialization—a creation instruction causes memory allocation; aninvocation instruction to invoke a constructor causes initialization.Each creation instruction corresponds to an invocation instruction togenerate respective objects. To generate an object of a specializedtype, a creation instruction supplies (a) an unspecialized type (thatdepends on an anchor constant and is therefore parametric) and (b) acontext for specializing the unspecialized type that results in thespecialized type. A memory space is allocated for the object based on amemory size and/or memory layout associated with the specialized type.The anchor value, resolved for the anchor constant, may indicate thememory size and/or memory layout associated with the specialized type.Further, an invocation instruction corresponding to the creationinstruction invokes a constructor defined in the unspecialized type, ora supertype of the underlying unspecialized type. If the constructordepends on the anchor constant (and/or other parametric constants), theinvocation instruction may further supply a context for invoking theconstructor.

In an embodiment, metadata of an object indicates a type of the object.Metadata of an object of a specialized type indicates the specializedtype of the object. The specialized type thus indicated may be referredto as a “recorded specialized type” for the object. In particular,metadata of an object of a specialized type may include a reference toan object (such as a class mirror) representing the specialized type.The anchor value, resolved for the anchor constant on which theunspecialized type depends, may indicate a reference to the objectrepresenting the specialized type.

One or more embodiments include accessing a parametric field of anobject within a particular context. An access instruction to access aparametric field of an object supplies a context. The context isassociated with a specialized type expected of the object. The expectedspecialized type is verified against a recorded specialized type of theobject. If a match is found, then the field is accessed based on amemory layout associated with the recorded specialized type. If no matchis found, then the field is accessed based on a different memory layout,such as a memory layout associated with a default context, or an erroris generated.

In an embodiment, a field is associated with a name and a type. Aninvariant type defines both the name and type of a field of theinvariant type. The invariant type is referred to as a “containing type”for the field. However, different types define the name and type of aspecialized field of a specialized type. The underlying unspecializedtype defines the name of the specialized field; the specialized typedefines the type of the specialized field. The specialized typespecializes the type of the specialized field within a particularcontext based on a type restriction applicable to the field with theparticular context. A value for the specialized field is type checked,converted, and/or casted based on the type restriction. Hence, thespecialized field may be referred to as having two “containing types,” a“specialized containing type” and an “unspecialized containing type.”

In an embodiment, access instructions of different instruction types areused to operate on values of different types. Such access instructionsmay include virtual machine instructions and/or machine code. Accessinstructions of different instruction types are used to access a sameparametric field within different contexts, because the parametric fieldis associated with different specialized field types within differentcontexts. An instruction type of an access instruction to a parametricfield is determined based on a type restriction applicable within aparticular context. As an example, an unspecialized type may specifythat a field is of the type Object, such that a memory space for thefield stores a reference (to a memory address in heap) rather than adata value. A specialized type may include a type restriction on thefield, restricting the field to values of int. An access instruction toaccess the field in the specialized type may be of the type int (suchas, iload). An access instruction to access the field in theunspecialized type may be of the type corresponding to references (suchas, aload).

One or more embodiments described in this Specification and/or recitedin the claims may not be included in this General Overview section.

2. Architectural Overview

FIG. 1 illustrates an example architecture in which techniques describedherein may be practiced. Software and/or hardware components describedwith relation to the example architecture may be omitted or associatedwith a different set of functionality than described herein. Softwareand/or hardware components, not described herein, may be used within anenvironment in accordance with one or more embodiments. Accordingly, theexample environment should not be constructed as limiting the scope ofany of the claims.

As illustrated in FIG. 1, a computing architecture 100 includes sourcecode files 101 which are compiled by a compiler 102 into class files 103representing the program to be executed. The class files 103 are thenloaded and executed by an execution platform 112, which includes aruntime environment 113, an operating system 111, and one or moreapplication programming interfaces (APIs) 110 that enable communicationbetween the runtime environment 113 and the operating system 111. Theruntime environment 113 includes a virtual machine 104 comprisingvarious components, such as a memory manager 105 (which may include agarbage collector), a class file verifier 106 to check the validity ofclass files 103, a class loader 107 to locate and build in-memoryrepresentations of classes, an interpreter 108 for executing the virtualmachine 104 code, and a just-in-time (JIT) compiler 109 for producingoptimized machine-level code.

In an embodiment, the computing architecture 100 includes source codefiles 101 that contain code that has been written in a particularprogramming language, such as Java, C, C++, C#, Ruby, Perl, and soforth. Thus, the source code files 101 adhere to a particular set ofsyntactic and/or semantic rules for the associated language. Forexample, code written in Java adheres to the Java LanguageSpecification. However, since specifications are updated and revisedover time, the source code files 101 may be associated with a versionnumber indicating the revision of the specification to which the sourcecode files 101 adhere. The exact programming language used to write thesource code files 101 is generally not critical.

In various embodiments, the compiler 102 converts the source code, whichis written according to a specification directed to the convenience ofthe programmer, to either machine or object code, which is executabledirectly by the particular machine environment, or an intermediaterepresentation (“virtual machine code/instructions”), such as bytecode,which is executable by a virtual machine 104 that is capable of runningon top of a variety of particular machine environments. The virtualmachine instructions are executable by the virtual machine 104 in a moredirect and efficient manner than the source code. Converting source codeto virtual machine instructions includes mapping source codefunctionality from the language to virtual machine functionality thatutilizes underlying resources, such as data structures. Often,functionality that is presented in simple terms via source code by theprogrammer is converted into more complex steps that map more directlyto the instruction set supported by the underlying hardware on which thevirtual machine 104 resides.

In general, programs are executed either as a compiled or an interpretedprogram. When a program is compiled, the code is transformed globallyfrom a first language to a second language before execution. Since thework of transforming the code is performed ahead of time; compiled codetends to have excellent run-time performance. In addition, since thetransformation occurs globally before execution, the code can beanalyzed and optimized using techniques such as constant folding, deadcode elimination, inlining, and so forth. However, depending on theprogram being executed, the startup time can be significant. Inaddition, inserting new code would require the program to be takenoffline, re-compiled, and re-executed. For many dynamic languages (suchas Java) which are designed to allow code to be inserted during theprogram's execution, a purely compiled approach may be inappropriate.When a program is interpreted, the code of the program is readline-by-line and converted to machine- level instructions while theprogram is executing. As a result, the program has a short startup time(can begin executing almost immediately), but the run-time performanceis diminished by performing the transformation on the fly. Furthermore,since each instruction is analyzed individually, many optimizations thatrely on a more global analysis of the program cannot be performed.

In some embodiments, the virtual machine 104 includes an interpreter 108and a JIT compiler 109 (or a component implementing aspects of both),and executes programs using a combination of interpreted and compiledtechniques. For example, the virtual machine 104 may initially begin byinterpreting the virtual machine instructions representing the programvia the interpreter 108 while tracking statistics related to programbehavior, such as how often different sections or blocks of code areexecuted by the virtual machine 104. Once a block of code surpasses athreshold (is “hot”), the virtual machine 104 invokes the JIT compiler109 to perform an analysis of the block and generate optimizedmachine-level instructions which replaces the “hot” block of code forfuture executions. Since programs tend to spend most time executing asmall portion of overall code, compiling just the “hot” portions of theprogram can provide similar performance to fully compiled code, butwithout the start-up penalty. Furthermore, although the optimizationanalysis is constrained to the “hot” block being replaced, there stillexists far greater optimization potential than converting eachinstruction individually. There are a number of variations on the abovedescribed example, such as tiered compiling.

In order to provide clear examples, the source code files 101 have beenillustrated as the “top level” representation of the program to beexecuted by the execution platform 112. Although the computingarchitecture 100 depicts the source code files 101 as a “top level”program representation, in other embodiments the source code files 101may be an intermediate representation received via a “higher level”compiler that processed code files in a different language into thelanguage of the source code files 101. Some examples in the followingdisclosure assume that the source code files 101 adhere to a class-basedobject-oriented programming language. However, this is not a requirementto utilizing the features described herein.

In an embodiment, compiler 102 receives as input the source code files101 and converts the source code files 101 into class files 103 that arein a format expected by the virtual machine 104. For example, in thecontext of the JVM, the Java Virtual Machine Specification defines aparticular class file format to which the class files 103 are expectedto adhere. In some embodiments, the class files 103 contain the virtualmachine instructions that have been converted from the source code files101. However, in other embodiments, the class files 103 may containother structures as well, such as tables identifying constant valuesand/or metadata related to various structures (classes, fields, methods,and so forth).

The following discussion assumes that each of the class files 103represents a respective “class” defined in the source code files 101 (ordynamically generated by the compiler 102/virtual machine 104). However,the aforementioned assumption is not a strict requirement and willdepend on the implementation of the virtual machine 104. Thus, thetechniques described herein may still be performed regardless of theexact format of the class files 103. In some embodiments, the classfiles 103 are divided into one or more “libraries” or “packages”, eachof which includes a collection of classes that provide relatedfunctionality. For example, a library may contain one or more classfiles that implement input/output (I/O) operations, mathematics tools,cryptographic techniques, graphics utilities, and so forth. Further,some classes (or fields/methods within those classes) may include accessrestrictions that limit their use to within a particularclass/library/package or to classes with appropriate permissions.

2.1 Example Class File Structure

FIG. 2 illustrates an example structure for a class file 200 in blockdiagram form according to an embodiment. In order to provide clearexamples, the remainder of the disclosure assumes that the class files103 of the computing architecture 100 adhere to the structure of theexample class file 200 described in this section. However, in apractical environment, the structure of the class file 200 will bedependent on the implementation of the virtual machine 104. Further, oneor more features discussed herein may modify the structure of the classfile 200 to, for example, add additional structure types. Therefore, theexact structure of the class file 200 is not critical to the techniquesdescribed herein. For the purposes of Section 2.1, “the class” or “thepresent class” refers to the class represented by the class file 200.

In FIG. 2, the class file 200 includes a constant table 201, fieldstructures 208, class structures 204, and method structures 209. In anembodiment, the constant table 201 is a data structure which, amongother functions, acts as a symbol table for the class. For example, theconstant table 201 may store data related to the various identifiersused in the source code files 101 such as type, scope, contents, and/orlocation. The constant table 201 has entries for value structures 202(representing constant values of type int, long, double, float, byte,string, and so forth), class information structures 203, name and typeinformation structures 205, field reference structures 206, and methodreference structures 207 derived from the source code files 101 by thecompiler 102. In an embodiment, the constant table 201 is implemented asan array that maps an index i to structure j. However, the exactimplementation of the constant table 201 is not critical.

In some embodiments, the entries of the constant table 201 includestructures which index other constant table 201 entries. For example, anentry for one of the value structures 202 representing a string may holda tag identifying its “type” as string and an index to one or more othervalue structures 202 of the constant table 201 storing char, byte or intvalues representing the ASCII characters of the string.

In an embodiment, field reference structures 206 of the constant table201 hold an index into the constant table 201 to one of the classinformation structures 203 representing the class defining the field andan index into the constant table 201 to one of the name and typeinformation structures 205 that provides the name and descriptor of thefield. Method reference structures 207 of the constant table 201 hold anindex into the constant table 201 to one of the class informationstructures 203 representing the class defining the method and an indexinto the constant table 201 to one of the name and type informationstructures 205 that provides the name and descriptor for the method. Theclass information structures 203 hold an index into the constant table201 to one of the value structures 202 holding the name of theassociated class.

The name and type information structures 205 hold an index into theconstant table 201 to one of the value structures 202 storing the nameof the field/method and an index into the constant table 201 to one ofthe value structures 202 storing the descriptor.

In an embodiment, class structures 204 (also referred to as “classmetadata”) includes metadata for the class, such as version number(s),number of entries in the constant pool, number of fields, number ofmethods, access flags (whether the class is public, private, final,abstract, etc.), an index to one of the class information structures 203of the constant table 201 that identifies the present class, an index toone of the class information structures 203 of the constant table 201that identifies the superclass (if any), and so forth.

In an embodiment, the field structures 208 represent a set of structuresthat identifies the various fields of the class. The field structures208 store, for each field of the class, accessor flags for the field(whether the field is static, public, private, final, etc.), an indexinto the constant table 201 to one of the value structures 202 thatholds the name of the field, and an index into the constant table 201 toone of the value structures 202 that holds a descriptor of the field.

In an embodiment, the method structures 209 represent a set ofstructures that identifies the various methods of the class. The methodstructures 209 store, for each method of the class, accessor flags forthe method (e.g. whether the method is static, public, private,synchronized, etc.), an index into the constant table 201 to one of thevalue structures 202 that holds the name of the method, an index intothe constant table 201 to one of the value structures 202 that holds thedescriptor of the method, and the virtual machine instructions thatcorrespond to the body of the method as defined in the source code files101.

In an embodiment, a descriptor represents a type of a field or method.For example, the descriptor may be implemented as a string adhering to aparticular syntax. While the exact syntax is not critical, a fewexamples are described below.

In an example where the descriptor represents a type of the field, thedescriptor identifies the type of data held by the field. In anembodiment, a field can hold a basic type, an object, or an array. Whena field holds a basic type, the descriptor is a string that identifiesthe basic type (e.g., “B”=byte, “C”=char, “D”=double, “F”=float,“I”=int, “J”=long int, etc.). When a field holds an object, thedescriptor is a string that identifies the class name of the object(e.g. “L ClassName”). “L” in this case indicates a reference, thus “LClassName” represents a reference to an object of class ClassName. Whenthe field is an array, the descriptor identifies the type held by thearray. For example, “[B” indicates an array of bytes, with “[”indicating an array and “B” indicating that the array holds the basictype of byte. However, since arrays can be nested, the descriptor for anarray may also indicate the nesting. For example, “[[L ClassName”indicates an array where each index holds an array that holds objects ofclass ClassName. In some embodiments, the ClassName is fully qualifiedand includes the simple name of the class, as well as the pathname ofthe class. For example, the ClassName may indicate where the file isstored in the package, library, or file system hosting the class file200.

In the case of a method, the descriptor identifies the parameters of themethod and the return type of the method. For example, a methoddescriptor may follow the general form “({ParameterDescriptor})ReturnDescriptor”, where the {ParameterDescriptor} is a list of fielddescriptors representing the parameters and the ReturnDescriptor is afield descriptor identifying the return type. For instance, the string“V” may be used to represent the void return type. Thus, a methoddefined in the source code files 101 as “Object m(int I, double d,Thread t) { . . . }” matches the descriptor “(I D L Thread) L Object”.

In an embodiment, the virtual machine instructions held in the methodstructures 209 include operations which reference entries of theconstant table 201. Using Java as an example, consider the followingclass:

class A {  int add12and13( ) {   return B.addTwo(12, 13);  } }

In the above example, the Java method add12and13 is defined in class A,takes no parameters, and returns an integer. The body of methodadd12and13 calls static method addTwo of class B which takes theconstant integer values 12 and 13 as parameters, and returns the result.Thus, in the constant table 201, the compiler 102 includes, among otherentries, a method reference structure that corresponds to the call tothe method B.addTwo. In Java, a call to a method compiles down to aninvoke command in the bytecode of the JVM (in this case invokestatic asaddTwo is a static method of class B). The invoke command is provided anindex into the constant table 201 corresponding to the method referencestructure that identifies the class defining addTwo “B”, the name ofaddTwo “addTwo”, and the descriptor of addTwo “(I I)I”. For example,assuming the aforementioned method reference is stored at index 4, thebytecode instruction may appear as “invokestatic #4”.

Since the constant table 201 refers to classes, methods, and fieldssymbolically with structures carrying identifying information, ratherthan direct references to a memory location, the entries of the constanttable 201 are referred to as “symbolic references”. One reason thatsymbolic references are utilized for the class files 103 is because, insome embodiments, the compiler 102 is unaware of how and where theclasses will be stored once loaded into the runtime environment 113. Aswill be described in Section 2.3, eventually the run-timerepresentations of the symbolic references are resolved into actualmemory addresses by the virtual machine 104 after the referenced classes(and associated structures) have been loaded into the runtimeenvironment and allocated concrete memory locations.

2.2 Example Virtual Machine Architecture

FIG. 3 illustrates an example virtual machine memory layout 300 in blockdiagram form according to an embodiment. In order to provide clearexamples, the remaining discussion will assume that the virtual machine104 adheres to the virtual machine memory layout 300 depicted in FIG. 3.In addition, although components of the virtual machine memory layout300 may be referred to as memory “areas”, there is no requirement thatthe memory areas are contiguous.

In the example illustrated by FIG. 3, the virtual machine memory layout300 is divided into a shared area 301 and a thread area 307. The sharedarea 301 represents an area in memory where structures shared among thevarious threads executing on the virtual machine 104 are stored. Theshared area 301 includes a heap 302 and a per-class area 303. In anembodiment, the heap 302 represents the run-time data area from whichmemory for class instances and arrays is allocated. In an embodiment,the per-class area 303 represents the memory area where the datapertaining to the individual classes are stored. In an embodiment, theper-class area 303 includes, for each loaded class, a run-time constantpool 304 representing data from the constant table 201 of the class,field and method data 306 (for example, to hold the static fields of theclass), and the method code 305 representing the virtual machineinstructions for methods of the class.

The thread area 307 represents a memory area where structures specificto individual threads are stored. In FIG. 3, the thread area 307includes thread structures 308 and thread structures 311, representingthe per-thread structures utilized by different threads. In order toprovide clear examples, the thread area 307 depicted in FIG. 3 assumestwo threads are executing on the virtual machine 104. However, in apractical environment, the virtual machine 104 may execute any arbitrarynumber of threads, with the number of thread structures scaledaccordingly.

In an embodiment, thread structures 308 includes program counter 309 andvirtual machine stack 310. Similarly, thread structures 311 includesprogram counter 312 and virtual machine stack 313. In an embodiment,program counter 309 and program counter 312 store the current address ofthe virtual machine instruction being executed by their respectivethreads.

Thus, as a thread steps through the instructions, the program countersare updated to maintain an index to the current instruction. In anembodiment, virtual machine stack 310 and virtual machine stack 313 eachstore frames for their respective threads that hold local variables andpartial results, and is also used for method invocation and return.

In an embodiment, a frame is a data structure used to store data andpartial results, return values for methods, and perform dynamic linking.A new frame is created each time a method is invoked. A frame isdestroyed when the method that caused the frame to be generatedcompletes. Thus, when a thread performs a method invocation, the virtualmachine 104 generates a new frame and pushes that frame onto the virtualmachine stack associated with the thread.

When the method invocation completes, the virtual machine 104 passesback the result of the method invocation to the previous frame and popsthe current frame off of the stack. In an embodiment, for a giventhread, one frame is active at any point. This active frame is referredto as the current frame, the method that caused generation of thecurrent frame is referred to as the current method, and the class towhich the current method belongs is referred to as the current class.

FIG. 4 illustrates an example frame 400 in block diagram form accordingto an embodiment. In order to provide clear examples, the remainingdiscussion will assume that frames of virtual machine stack 310 andvirtual machine stack 313 adhere to the structure of frame 400.

In an embodiment, frame 400 includes local variables 401, operand stack402, and run-time constant pool reference table 403. In an embodiment,the local variables 401 are represented as an array of variables thateach hold a value, for example, Boolean, byte, char, short, int, float,or reference. Further, some value types, such as longs or doubles, maybe represented by more than one entry in the array. The local variables401 are used to pass parameters on method invocations and store partialresults. For example, when generating the frame 400 in response toinvoking a method, the parameters may be stored in predefined positionswithin the local variables 401, such as indexes 1-N corresponding to thefirst to Nth parameters in the invocation.

In an embodiment, the operand stack 402 is empty by default when theframe 400 is created by the virtual machine 104. The virtual machine 104then supplies instructions from the method code 305 of the currentmethod to load constants or values from the local variables 401 onto theoperand stack 402. Other instructions take operands from the operandstack 402, operate on them, and push the result back onto the operandstack 402. Furthermore, the operand stack 402 is used to prepareparameters to be passed to methods and to receive method results. Forexample, the parameters of the method being invoked could be pushed ontothe operand stack 402 prior to issuing the invocation to the method. Thevirtual machine 104 then generates a new frame for the method invocationwhere the operands on the operand stack 402 of the previous frame arepopped and loaded into the local variables 401 of the new frame. Whenthe invoked method terminates, the new frame is popped from the virtualmachine stack and the return value is pushed onto the operand stack 402of the previous frame.

In an embodiment, the run-time constant pool reference table 403contains a reference to the run-time constant pool 304 of the currentclass. The run-time constant pool reference table 403 is used to supportresolution. Resolution is the process whereby symbolic references in theconstant pool 304 are translated into concrete memory addresses, loadingclasses as necessary to resolve as-yet-undefined symbols and translatingvariable accesses into appropriate offsets into storage structuresassociated with the run-time location of these variables.

2.3 Loading, Linking, And Initializing

In an embodiment, the virtual machine 104 dynamically loads, links, andinitializes classes. Loading is the process of finding a class with aparticular name and creating a representation from the associated classfile 200 of that class within the memory of the runtime environment 113.For example, creating the run-time constant pool 304, method code 305,and field and method data 306 for the class within the per-class area303 of the virtual machine memory layout 300. Linking is the process oftaking the in-memory representation of the class and combining it withthe run-time state of the virtual machine 104 so that the methods of theclass can be executed. Initialization is the process of executing theclass constructors to set the starting state of the field and methoddata 306 of the class and/or create class instances on the heap 302 forthe initialized class.

The following are examples of loading, linking, and initializingtechniques that may be implemented by the virtual machine 104. However,in many embodiments the steps may be interleaved, such that an initialclass is loaded, then during linking a second class is loaded to resolvea symbolic reference found in the first class, which in turn causes athird class to be loaded, and so forth. Thus, progress through thestages of loading, linking, and initializing can differ from class toclass. Further, some embodiments may delay (perform “lazily”) one ormore functions of the loading, linking, and initializing process untilthe class is actually required. For example, resolution of a methodreference may be delayed until a virtual machine instruction invokingthe method is executed. Thus, the exact timing of when the steps areperformed for each class can vary greatly between implementations.

To begin the loading process, the virtual machine 104 starts up byinvoking the class loader 107 which loads an initial class. Thetechnique by which the initial class is specified will vary fromembodiment to embodiment. For example, one technique may have thevirtual machine 104 accept a command line argument on startup thatspecifies the initial class.

To load a class, the class loader 107 parses the class file 200corresponding to the class and determines whether the class file 200 iswell-formed (meets the syntactic expectations of the virtual machine104). If not, the class loader 107 generates an error. For example, inJava the error might be generated in the form of an exception which isthrown to an exception handler for processing. Otherwise, the classloader 107 generates the in-memory representation of the class byallocating the run-time constant pool 304, method code 305, and fieldand method data 306 for the class within the per-class area 303.

In some embodiments, when the class loader 107 loads a class, the classloader 107 also recursively loads the super-classes of the loaded class.For example, the virtual machine 104 may ensure that the super-classesof a particular class are loaded, linked, and/or initialized beforeproceeding with the loading, linking and initializing process for theparticular class.

During linking, the virtual machine 104 verifies the class, prepares theclass, and performs resolution of the symbolic references defined in therun-time constant pool 304 of the class.

To verify the class, the virtual machine 104 checks whether thein-memory representation of the class is structurally correct. Forexample, the virtual machine 104 may check that each class except thegeneric class Object has a superclass, check that final classes have nosub-classes and final methods are not overridden, check whether constantpool entries are consistent with one another, check whether the currentclass has correct access permissions for classes/fields/structuresreferenced in the constant pool 304, check that the virtual machine 104code of methods will not cause unexpected behavior (e.g. making sure ajump instruction does not send the virtual machine 104 beyond the end ofthe method), and so forth. The exact checks performed duringverification are dependent on the implementation of the virtual machine104. In some cases, verification may cause additional classes to beloaded, but does not necessarily require those classes to also be linkedbefore proceeding. For example, assume Class A contains a reference to astatic field of Class B. During verification, the virtual machine 104may check Class B to ensure that the referenced static field actuallyexists, which might cause loading of Class B, but not necessarily thelinking or initializing of Class B. However, in some embodiments,certain verification checks can be delayed until a later phase, such asbeing checked during resolution of the symbolic references. For example,some embodiments may delay checking the access permissions for symbolicreferences until those references are being resolved.

To prepare a class, the virtual machine 104 initializes static fieldslocated within the field and method data 306 for the class to defaultvalues. In some cases, setting the static fields to default values maynot be the same as running a constructor for the class. For example, theverification process may zero out or set the static fields to valuesthat the constructor would expect those fields to have duringinitialization.

During resolution, the virtual machine 104 dynamically determinesconcrete memory address from the symbolic references included in therun-time constant pool 304 of the class. To resolve the symbolicreferences, the virtual machine 104 utilizes the class loader 107 toload the class identified in the symbolic reference (if not alreadyloaded). Once loaded, the virtual machine 104 has knowledge of thememory location within the per-class area 303 of the referenced classand its fields/methods. The virtual machine 104 then replaces thesymbolic references with a reference to the concrete memory location ofthe referenced class, field, or method. In an embodiment, the virtualmachine 104 caches resolutions to be reused in case the sameclass/name/descriptor is encountered when the virtual machine 104processes another class. For example, in some cases, class A and class Bmay invoke the same method of class C. Thus, when resolution isperformed for class A, that result can be cached and reused duringresolution of the same symbolic reference in class B to reduce overhead.

In some embodiments, the step of resolving the symbolic referencesduring linking is optional. For example, an embodiment may perform thesymbolic resolution in a “lazy” fashion, delaying the step of resolutionuntil a virtual machine instruction that requires the referencedclass/method/field is executed.

During initialization, the virtual machine 104 executes the constructorof the class to set the starting state of that class. For example,initialization may initialize the field and method data 306 for theclass and generate/initialize any class instances on the heap 302created by the constructor. For example, the class file 200 for a classmay specify that a particular method is a constructor that is used forsetting up the starting state. Thus, during initialization, the virtualmachine 104 executes the instructions of that constructor.

In some embodiments, the virtual machine 104 performs resolution onfield and method references by initially checking whether thefield/method is defined in the referenced class. Otherwise, the virtualmachine 104 recursively searches through the super-classes of thereferenced class for the referenced field/method until the field/methodis located, or the top-level superclass is reached, in which case anerror is generated.

3. Parametric Constants

FIGS. 5A-5B illustrate state machines for constants, and in particular,parametric constants, according to an embodiment.

In one or more embodiments, a state machine 500 for a constant includestwo states, an unresolved state 502 and a resolution state 504.

Initially, a constant is prepared. Preparation may be performed duringpreparation of a type defining the constant. Alternatively, preparationmay be triggered by other events. Once the constant is prepared, theconstant assumes an unresolved state 502.

Next, the constant is resolved. Resolution may be performed based on abootstrap method corresponding to the constant. Alternatively,resolution may be performed based on another method, which may bedefined by the runtime environment. Once the constant is resolved, theconstant assumes a resolution state 504. The process of resolutiondetermines a resolved value, or an error, for the resolution state 504.A resolved value is stored in a heap.

Within a state machine 500, at most one resolution state 504 correspondsto each unresolved state 502. At most one resolution step corresponds toeach preparation step. Further, no other states follow after the objectreaches a resolution state 504. Once the resolution state 504 isreached, the resolution state 504 is permanent for the constant.

Any value (whether a resolved value or an error) determined for aresolution state 504 is unchangeable for the resolution state 504.Hence, the resolved value or error is permanent for the resolution state504 of the constant.

In one or more embodiments, at most one state machine 500 is associatedwith an invariant constant. Therefore, an invariant constant representsat most one value. In contrast, multiple state machines 516 a-b may beassociated with a parametric constant 506. As illustrated, for example,state machine 516 a includes unresolved state 512 a transitioning toresolution state 514 a ; state machine 516 b includes unresolved state512 b transitioning to resolution state 514 b. State machine 516 a-bassociated with a same parametric constant 506 correspond to differentcontexts. State machines 516 a-b and contexts have a one-to-manyrelationship: each state machine relates to one or more contexts; eachcontext relates to a single state machine. Based on the multiple statemachines 516 a-b, a parametric constant 506 may represent differentvalues across different contexts, but at most one value within eachcontext. The term “constant” as used herein may refer to an invariantconstant and/or parametric constant.

FIG. 6 illustrates runtime data areas storing information for constants,including parametric constants, according to an embodiment.

In one or more embodiments, runtime data areas 600 of a virtual machineinclude one or more per-type runtime constant pools 602 a -b, and a heap608. A per-type runtime constant pool 602 a -b is similar to a runtimeconstant pool 304 of FIG. 3. A heap 608 is similar to a heap 302 of FIG.3.

In one or more embodiments, a single per-type runtime constant pool(such as any of per-type runtime constant pools 602 a -b) corresponds toeach type loaded in a virtual machine. A per-type runtime constant poolcorresponding to a type stores entries for both invariant constants andparametric constants defined by the type. Each resolved entry within aper-type runtime constant pool refers to a respective resolved value(such as any of resolved values 610-616 b).

In an embodiment, a per-type runtime constant pool includes a singleinvariant runtime constant pool 604, storing entries for invariantconstants defined by the type.

In an embodiment, a per-type runtime constant pool includes zero or moreparametric runtime constant pools 606 a-b, storing entries forparametric constants defined by the type.

In an embodiment, a reference to a per-type runtime constant pool withina particular context may include multiple components: a reference to theinvariant runtime constant pool and a reference to a parametric runtimeconstant pool that is relevant to the particular context.

In one or more embodiments, a set of parametric runtime constant pools606 a-b stores respective resolution states of a particular parametricconstant. A parametric runtime constant pool storing a resolution statethat is relevant to one or more contexts is also associated with thesame one or more contexts. Parametric runtime constant pools 606 a-b andcontexts have a one-to-many relationship: each parametric runtimeconstant pool relates to one or more contexts; each context relates to asingle parametric runtime constant pool.

In one or more embodiments, a parametric runtime constant pool storesresolution states for multiple co-dependent constants. A constant thatdepends on a parametric constant is also itself parametric. (Furtherdescriptions regarding dependency are included below with reference toFIG. 13.)

In one or more embodiments, a particular parametric constant correspondsto the same index in each of a set of parametric runtime constant pools606 a -b. However, the respective entries at the same index in differentparametric runtime constant pools 606 a-b point to different resolvedvalues 614 a-b within a heap 608.

As illustrated, for example, a type defines four constants:

-   Index #1: [a first invariant constant]-   Index #2: [a second invariant constant]-   Index #3: [a first parametric constant]-   Index #4: [a second parametric constant that depends on the first    parametric constant]

A per-type runtime constant pool 602 a includes an invariant runtimeconstant pool 604. The invariant runtime constant pool 604 storesentries for Index #1 and Index #2. Index #1 points to resolved value 610within a heap 608. The resolved value 610 referenced by Index #1 is theresolved value for the first invariant constant corresponding to Index#1.

The per-type runtime constant pool 602 a includes a parametric runtimeconstant pool 606 a, associated with one context, and another parametricruntime constant pool 606 b, associated with another context.

The parametric runtime constant pool 606 a stores entries for Index #3and Index #4. Index #3 points to resolved value 614 a within the heap608. The resolved value 614 a referenced by Index #3 is the resolvedvalue for the first parametric constant corresponding to Index #3.Further, the resolved value 614 a is the resolved value for the firstparametric constant within the context associated with the parametricruntime constant pool 606 a.

The parametric runtime constant pool 606 b also stores entries for Index#3 and Index #4. The resolved value 614 b referenced by Index #3 is theresolved value for the first parametric constant corresponding to Index#3. Further, the resolved value 614 b is the resolved value for thefirst parametric constant within the context associated with theparametric runtime constant pool 606 b. Resolved values 614 a-b may besame or different.

FIG. 7 illustrates a call stack storing information for constants,including parametric constants, according to an embodiment.

In one or more embodiments, a call stack 700 includes stack frames 702 a-d. A stack frame (such as stack frame 702 b, 702 d) may correspond toexecution of an application programming interface (API) point thatdepends on a parametric constant. A caller to the API point supplies acontext for the execution of the API point. The contextual informationis passed from a caller stack frame to a callee stack frame. The calleestack frame therefore is associated with the context supplied by thecaller stack frame.

In an example, a type Bicycle defines a first parametric constant and afirst API point dependent on the first parametric constant. Another typeBoat defines a second parametric constant and a second API pointdependent on the second parametric constant.

Stack frame 702 a corresponds to execution of a first caller method. Thefirst caller method calls the first API point in Bicycle. The firstcaller method supplies a context 710 a for the execution of the firstAPI point.

Based on the call to the first API point, a new stack frame 702 b ispushed onto the call stack 700. Stack frame 702 a corresponds toexecution of the first API point. Information passed from stack frame702 a to stack frame 702 b includes arguments to the first API point aswell as the context 710 a. Context 710 a may be referred to as a“context” for stack frame 702 b. Execution of the first API pointinvokes a second caller method.

Based on the call to the second caller method, a new stack frame 702 cis pushed onto the call stack 700. Stack frame 702 c corresponds toexecution of the second caller method. The second caller method callsthe second API point in Boat. The second caller method supplies acontext 710 c for the execution of the second API point.

Based on the call to the second API point, a new stack frame 702 d ispushed onto the call stack 700. Stack frame 702 d corresponds toexecution of the second API point. Information passed from stack frame702 c to stack frame 702 d includes arguments to the second API point aswell as the context 710 c. Context 710 c may be referred to as a“context” for stack frame 702 d.

In one or more embodiments, a per-type runtime constant pool is relevantto each stack frame. Potentially multiple per-type runtime constantpools may exist in a runtime environment, each corresponding to arespective type loaded in the runtime environment. A per-type runtimeconstant pool relevant to a particular stack frame corresponding toexecution of a particular API point in a particular type is the per-typeruntime constant pool corresponding to the particular type.

In one or more embodiments, each stack frame 702 a -d is associated witha runtime constant pool reference pointing to the relevant per-typeruntime constant pool. Where a stack frame 702 b corresponds toexecution of an API point that depends on a parametric constant, theruntime constant pool reference includes an invariant runtime constantpool reference 706 b and a parametric runtime constant pool reference708 b. The invariant runtime constant pool reference 706 b points to theinvariant runtime constant pool of the type including the API point. Theparametric runtime constant pool reference 708 b points to theparametric runtime constant pool that is (a) of the type including theAPI point and (b) associated with the same context as the stack frame702 b.

Referring to the above example, stack frame 702 a is associated withruntime constant pool reference 703 a ; stack frame 702 b is associatedwith runtime constant pool reference 703 b ; stack frame 702 c isassociated with runtime constant pool reference 703 c ; and stack frame702 d is associated with runtime constant pool reference 703 d.

Stack frame 702 b corresponds to execution of the first API point inBicycle. Runtime constant pool reference 702 b includes invariantruntime constant pool reference 706 b and parametric runtime constantpool reference 708 b. Invariant runtime constant pool reference 706 bpoints to the invariant runtime constant pool associated with the typeBicycle. Parametric runtime constant pool reference 708 b points to oneof a set of parametric runtime constant pools associated with the typeBicycle. The referenced parametric runtime constant pool is associatedwith the same context 710 a as stack frame 702 b.

Stack frame 702 d corresponds to execution of the second API point inBoat. Runtime constant pool reference 702 d includes invariant runtimeconstant pool reference 706 d and parametric runtime constant poolreference 708 d. Invariant runtime constant pool reference 706 d pointsto the invariant runtime constant pool associated with the type Boat.Parametric runtime constant pool reference 708 d points to one of a setof parametric runtime constant pools associated with the type Boat. Thereferenced parametric runtime constant pool is associated with the samecontext 710 c as stack frame 702 d.

In one or more embodiments, operations executed within each stack frame702 a -d rely on the stack frame's runtime constant pool reference toaccess relevant resolution states of relevant constants. Operationswithin a stack frame cannot access resolution states outside of theper-type runtime constant pool referenced by the stack frame's runtimeconstant pool reference.

Referring to the above example, stack frame 702 b corresponds toexecution of the first API point in Bicycle. The type Bicycle isassociated with multiple parametric runtime constant pools, each storingentries for a same set of parametric constants. Stack frame 702 bincludes a runtime constant pool reference 703 b, which includesinvariant runtime constant pool reference 706 b and parametric runtimeconstant pool reference 708 b. Operations executed within stack frame702 b may access resolution states within the parametric runtimeconstant pool referenced by parametric runtime constant pool reference708 b ; however, operations executed within stack frame 702 b cannotaccess resolution states within other parametric runtime constant poolsassociated with the same set of parametric constants.

Stack frame 702 d corresponds to execution of the second API point inBoat. The type Boat is associated with multiple parametric runtimeconstant pools, each storing entries for a same set of parametricconstants. Stack frame 702 d includes a runtime constant pool reference703 d, which includes invariant runtime constant pool reference 706 dand parametric runtime constant pool reference 708 d. Operationsexecuted within stack frame 702 d may access resolution states withinthe parametric runtime constant pool referenced by parametric runtimeconstant pool reference 708 d ; however, operations executed withinstack frame 702 d cannot access resolution states within otherparametric runtime constant pools associated with the same set ofparametric constants.

In one or more embodiments, stack frames corresponding to differentexecutions of one or more API points that are dependent on a sameparametric constant within a same context reference a same parametricruntime constant pool. Accordingly, every execution of API pointsdependent on a same parametric constant within a same context referencesthe same resolution state for the parametric constant. Further, theresolved value is unchangeable for the resolution state as describedabove. Therefore, the resolved value is constant and permanent for theparametric constant across different executions of the API points withinthe same context. Conversely, different executions of API pointsdependent on a same parametric constant in different contexts referencedifferent resolution states for the parametric constant. Therefore,different executions in different contexts may access different resolvedvalues for the different resolution states of the same parametricconstant.

4. Parametric Application Programming Interface (API) Points

FIG. 8 illustrates a class file including one or more parametric APIpoints according to an embodiment.

In one or more embodiments, a class file 800 for a type includes variousinformation structures for different components of the type, including aclass structure 802, one or more field structures 804, and one or moremethod structures 806. Further, a class file 800 includes a constantpool table 832 and a bootstrap method table 838. A class file 800 issimilar to a class file 200 of FIG. 2. A class structure 802 is similarto a class structure 204 of FIG. 2. A field structure 804 is similar toa field structure 208 of FIG. 2. A method structure 806 is similar to amethod structure 209 of FIG. 2. A constant pool table 832 is similar toa constant table 201 of FIG. 2.

In one or more embodiments, an information structure for an applicationprogramming interface (API) point is associated with a descriptor. Theterm “API point” as used herein refers to an interface, a class, afield, a method, and/or a constructor. As illustrated, a field structure804 for a field in the type is associated with a descriptor 812; amethod structure 806 for a method in the type is associated with adescriptor 822.

A descriptor for an API point indicates a name and type of the APIpoint. As illustrated, descriptor 812 indicates name 814 and type 816for field structure 804; descriptor 822 indicates name 824 and type 826for method structure 806. A descriptor for a field indicates a name ofthe field and a type of the field. A type of a field may also bereferred to specifically as a “field type.” A descriptor for a methodindicates a name of the method, and one or more types of a method returnvalue, and zero or more method parameters.

In one or more embodiments, an information structure for an API point isassociated with an attributes table. As illustrated, a class structure802 for a type is associated with an attributes table 808; a fieldstructure 804 for a field in the type is associated with an attributestable 818; a method structure 806 for a method in the type is associatedwith an attributes table 828. An attributes table (such as any ofattributes tables 808, 818, 828) includes zero or more attributesassociated with the corresponding API point.

In one or more embodiments, a parametric attribute (such as any ofparametric attributes 810, 820, 830) is a kind of attribute that marksan API point as parametric and therefore specializable with respect to aparticular context. If a parametric attribute is present in aninformation structure for an API point, then the API point isparametric. If no parametric attribute is present in an informationstructure for an API point, then the API point is invariant.

A parametric attribute includes an attribute name index, an attributelength, and a parametric constant reference. The attribute name indexindicates an identifier for the parametric attribute (such as thestring, “Parametric”). The attribute length indicates a length of thesubsequent information in the parametric attribute. The parametricconstant reference points to a constant (for example, the parametricconstant reference is an index into the constant pool table 832) that isparametric. (The referenced parametric constant may be a specializationanchor constant, which is further discussed below with reference tospecialization anchor constant 906 of FIG. 9.) Based on the referencefrom a parametric attribute, a dependency from the API point on theparametric constant is established. As illustrated, a parametricattribute 810 associated with a class structure 802 for a typereferences a parametric constant 834 within a constant pool table 832,thereby indicating that the type depends on the parametric constant 834.A parametric attribute 820 associated with a field structure 804 for afield in the type references the parametric constant 834 within theconstant pool table 832, thereby indicating that the field depends onthe parametric constant 834. A parametric attribute 830 associated witha method structure 806 for a method in the type references a parametricconstant 836 within the constant pool table 832, thereby indicating thatthe method depends on the parametric constant 836.

In one or more embodiments, a constant pool table 832 of a type includesa set of entries corresponding to respective constants (such asparametric constants 834-836) defined by the type. Each entry is aninformation structure for a corresponding constant. An informationstructure for a constant includes a tag and an information array. Thetag indicates a kind of the corresponding constant. The informationarray includes information about the corresponding constant. Theinformation array may include, for example, a reference to a bootstrapmethod for the corresponding constant.

In one or more embodiments, a bootstrap method (such as any of bootstrapmethods 840-842) for a constant is a method configured to bring aconstant from an unresolved state to a resolution state. A bootstrapmethod is configured to determine a resolved value (or an error) for aresolution state of a constant. As illustrated, for example, bootstrapmethod 840 corresponds to parametric constant 834; bootstrap method 842corresponds to parametric constant 836.

FIG. 9 illustrates information structures for a specialization anchorconstant and a specialization linkage constant according to anembodiment.

In one or more embodiments, a callee 902 defines (or inherits from asupertype) an API point 904. Further the callee 902 defines aspecialization anchor constant 906. A class file for the callee 902 may,for example, include an information structure for the API point 904, andan entry in a constant pool table for the specialization anchor constant906. The term “caller” as used herein may refer to a type or methodincluding a call to an API point, or may refer to the API point itself.A “caller” and a “callee” may reside in a same type or different types.

In one or more embodiments, a callee 902 specifies a dependency from anAPI point 904 on a specialization anchor constant 906. The API point 904may, for example, be associated with a parametric attribute thatincludes a parametric constant reference to the specialization anchorconstant 906. Though not illustrated, multiple API points defined by thecallee 902 may depend on the same specialization anchor constant 906.Additionally or alternatively, another API point defined by the callee902 may depend on another specialization anchor constant defined by thecallee 902.

In one or more embodiments, a specialization anchor constant 906 (alsoreferred to herein as an “anchor constant”) is a parametric constantused to declare a distinct degree of freedom of parametricity forparametric API points defined by the type. An information structure fora specialization anchor constant 906 includes a tag, an anchor kind, anda bootstrap method reference. The tag indicates an identifier for theanchor constant. The anchor kind indicates a kind of parametricityassociated with the anchor constant. The bootstrap method referencepoints to a bootstrap method 908 (for example, the bootstrap methodreference is an index into a bootstrap method table in the callee 902).

In an embodiment, an anchor constant 906 may be associated with one ofvarious kinds of parametricity. A “class parametricity” involves adegree of freedom that applies to a class as a whole. Additionally oralternatively, a “method parametricity” involves a degree of freedomthat applies to a method. Each anchor constant of the “methodparametricity” kind varies independently from other anchor constants inthe same type. Additionally or alternatively, a “method and classparametricity” involves a degree of freedom that applies to a method. Ananchor constant of the “method and class parametricity” kind, bydefinition, depends on another anchor constant of the “classparametricity” kind in the same type.

In an embodiment, a bootstrap method 908 is configured to bring ananchor constant 906 from an unresolved state to a resolution state. Abootstrap method 908 returns different resolved values for differentresolution states of an anchor constant 906 within different contexts.

A bootstrap method 908 accepts as an argument a selector value 918indicating a callee's 902 proposed specialization for the API point 904.A bootstrap method 908 may include operations configured to verifywhether the selector value 918 is valid for the anchor constant 906. Ifthe selector value 918 is valid, the bootstrap method 908 determines aset of specialization decisions for the API point 904 based on theselector value 918. The bootstrap method returns a resolved value for aresolution state of the anchor constant 906; the resolved valueindicates the set of specialization decisions and may be referred to asan “anchor value,” which is further discussed below with reference toanchor value 1302 of FIG. 13. However if the selector value 918 s notvalid, the bootstrap method 908 returns an error for the resolutionstate of the anchor constant 906.

In one or more embodiments, a caller 910 defines a call site 912 and aspecialization linkage constant 914. A class file for the caller 910 mayinclude, for example, an instruction that calls an API point 904 of acallee 902. The class file for the caller 910 may further include anentry in a constant pool table for the specialization linkage constant914. The term “callee” as used herein may refer to a type or methodincluding an API point being called, or may refer to the API pointitself. A “caller” and A “callee” may reside in a same type or differenttypes.

In one or more embodiments, a caller 910 specifies an operand of aninstruction at a call site 912. The operand may be either a reference toan API point 904 being called, or a specialization linkage constant 914.Where the operand is a specialization linkage constant 914, anassociation between the call site 912 and the specialization linkageconstant 914 is established. Though not illustrated, multiple call sitesdefined by the caller 910 may be associated with the same specializationlinkage constant 914. Additionally or alternatively, another call sitedefined by the caller 910 may be associated with a differentspecialization linkage constant defined by the caller 910.

In one or more embodiments, a specialization linkage constant 914 (alsoreferred to as a “linkage constant”) is a constant used to addparametric information to a call to an API point 904. An informationstructure for a specialization linkage constant 914 includes a tag, anAPI point reference 916, and a selector value 916 applicable to a callto the referenced API point 904. The tag indicates an identifier for thelinkage constant. The API point reference 916 points to the API point904 being called (for example, the API point reference is an index intothe constant pool table in the caller 910 that corresponds to the APIpoint 904, which may be referred to as a “symbolic reference” to the APIpoint 904). The selector value 916 indicates a context for the call tothe API point 904, as further described below.

In one or more embodiments, a selector value 916 is another constantdefined by the caller 910. The selector value 916 may be previouslyvalidated by the callee 902, or may be unvalidated. If the selectorvalue 916 is previously validated, then the selector value 916 is ananchor value, determined as a resolved value for a resolution state ofthe anchor constant 906. If the selector value 916 is unvalidated, thenthe selector value 916 includes information intended to request somespecialization of the API point 904. For example, a callee requestingcreation of List<Point>may reference a selector value that referencesthe class List and the class Point. An unvalidated selector value 916 isinput as an argument into the bootstrap method 908 for resolving theanchor constant 906.

FIGS. 10A-10B illustrate state machines for a specialization anchorconstant and a specialization linkage constant according to anembodiment.

In one or more embodiments, a specialization anchor constant 1002 may beassociated with one or more state machines 1008. A state machine 1008for an anchor constant 1002 includes two states: an unresolved state1004 and a resolution state 1006. The anchor constant 1002 is firstprepared and enters the unresolved state 1004. The anchor constant 1002is then resolved (for example, via execution of a bootstrap method) andenters the resolution state 1006. Once in the resolution state 1006, theanchor constant 1002 cannot assume any other states within the statemachine 1008. A resolved value determined for the resolution state 1006is not changeable.

In one or more embodiments, unless the resolution state 1006 indicatesan error, the resolution state 1006 indicates a set of specializationdecisions for the anchor constant 1002 within a particular context—thecontext associated with a selector value provided by a callee. The setof specialization decisions may be embodied in an “anchor value,” whichis further discussed below with reference to anchor value 1302 of FIG.13. In an embodiment, an anchor constant 906 is a loadable constant. Theloadable value of an anchor constant 906 includes at least a subset ofinformation indicated by the anchor value.

In one or more embodiments, an anchor constant 1002 is associated with(a) a state machine within a default context and (b) zero or moreadditional state machines within contexts supplied by callers to APIpoints dependent on the anchor constant 1002. Each additional statemachine is associated with a call to a dependent API point within a newcontext. However, the state machine within the default context isassociated with an initial preparation and resolution of a type definingthe anchor constant 1002. Within the state machine within the defaultcontext, the resolution state for the anchor constant 1002 is referredto as a “default resolution state,” and the anchor value resolved forthe anchor constant 1002 is referred to as a “default anchor value.”

In one or more embodiments, a state machine 1018 for a specializationlinkage constant 1012 includes two states: an unresolved state 1014 anda resolution state 1016. The linkage constant 1012 is first prepared andenters the unresolved state 1014. The linkage constant 1012 is thenresolved and enters the resolution state 1016. Once in the resolutionstate 1016, the linkage constant 1012 cannot assume any other stateswithin the state machine 1018. A resolved value determined for theresolution state 1016 is not changeable.

In one or more embodiments, unless the resolution state 1016 indicatesan error, the resolution state 1016 indicates an API point and an anchorvalue determined for the anchor constant 1002 on which the API pointdepends. A resolved value for the resolution state 1016 includes adirect reference (rather than a symbolic reference) to the API point 904and a direct reference to the anchor value. As described above, theanchor value embodies a set of specialization decisions by the calleebased on a selector value provided by the caller.

FIG. 11 illustrates storage of an anchor value included in resolvedvalues for resolution states of a specialization anchor constant and aspecialization linkage constant according to an embodiment.

In one or more embodiments, runtime data areas 1100 of a virtual machineincludes a per-type runtime constant pool 1102 associated with a callee,a per-type runtime constant pool 1104 associated with a caller, and aheap 1106. In an alternative embodiment, the callee and the caller mayreside in a common type, and therefore be associated with the sameper-type runtime constant pool.

In one or more embodiments, a callee defines an anchor constant. Theanchor constant is within a constant pool table of the callee. Theanchor constant occupies an entry in the callee's per-type constant pool1102. As illustrated, for example, an anchor constant occupies Index #3.

In one or more embodiments, a resolved value for a resolution state ofan anchor constant indicates an anchor value 1108. Therefore, aresolution state of the anchor constant includes a reference to theanchor value 1108 stored within the heap 1106.

In one or more embodiments, a caller defines a linkage constant. Thelinkage constant is within a constant pool table of the caller. Thelinkage constant occupies an entry in the caller's per-type constantpool 1102. As illustrated, for example, an linkage constant occupiesIndex #7.

In one or more embodiments, a resolved value for a resolution state ofan linkage constant indicates an anchor value 1108 (and an API pointspecialized based on the anchor value 1108). Therefore, a resolutionstate of the linkage constant includes a reference to the anchor value1108 stored within the heap 1106. Hence, each of the anchor constant andthe linkage constant is resolved to point to a same anchor value 1108.

FIG. 12A illustrates an unspecialized type and a specialized type thatis the unspecialized type specialized within a particular contextaccording to an embodiment.

In one or more embodiments, an unspecialized type (such as anunspecialized containing type 1202, an unspecialized field type 1208, anunspecialized method parameter type (not illustrated), and/or anunspecialized method return value type (not illustrated)) is associatedwith a class structure (such as class structure 204 of FIG. 2). A classstructure for an unspecialized type includes one or more of:

-   a definition of a constant pool table corresponding to the    unspecialized type;-   an identification of a superclass (a default value, such as zero,    may indicate a default class, such as Object);-   identification(s) of one or more superinterfaces;-   a fields table;-   a methods table; and/or-   an attributes table.

In an embodiment, each entry in a fields table is a field structure(such as field structure 208 of FIG. 2). A field structure provides acomplete description of a field (such as field 1206) in theunspecialized type. A fields table includes fields declared by theunspecialized type; a fields table does not necessarily include fieldsthat are inherited from superclasses or superinterfaces.

In an embodiment, a field structure for a field 1206 indicates adescriptor for the field 1206. The descriptor indicates a name andunspecialized field type 1208 of the field.

In an embodiment, a field 1206 is declared, in source code, as beingparametrized over a type parameter T. During compilation into virtualmachine instructions, the type parameter T is erased to become an upperbound type associated with the type parameter T. The upper bound type ofthe type parameter T is either specified in the source code, or assumedto be Object. The upper bound type of the type parameter T is used asthe unspecialized field type 1208 of the field 1206. Therefore, thedescriptor of the field 1206 indicates the upper bound type of the typeparameter T as the unspecialized field type 1208 of the field 1206.

In an embodiment, each entry in a methods table is a method structure(such as method structure 209 of FIG. 2). A method structure provides acomplete description of a method in the unspecialized type. A methodstable includes methods declared by the unspecialized type; a methodstable does not necessarily include methods that are inherited fromsuperclasses or superinterfaces.

In an embodiment, a method structure for a method indicates a descriptorfor the method. The descriptor indicates a name of the method,unspecialized types of method parameters (if any), and an unspecializedtype of a method return value (if any).

In an embodiment, a method parameter and/or method return value of amethod is declared, in source code, as being parametrized over a typeparameter T. During compilation into virtual machine instructions, thetype parameter T is erased to become an upper bound type associated withthe type parameter T. The upper bound type of the type parameter T iseither specified in the source code, or assumed to be Object. The upperbound type of the type parameter T is used as an unspecialized methodparameter type and/or unspecialized method return value type associatedwith the method. Therefore, the descriptor of the method indicates theupper bound type of the type parameter T as the unspecialized methodparameter type and/or unspecialized method return value type.

In one or more embodiments, a specialized type (such as a specializedcontaining type 1214, a specialized field type 1212, a specializedmethod parameter type (not illustrated), and/or a specialized methodreturn value type (not illustrated)) is an unspecialized typespecialized within a particular context 1204.

As illustrated, for example, an unspecialized containing type 1202defines a field 1206 and specifies that the field 1206 is of anunspecialized field type 1208. In particular, the unspecializedcontaining type 1202 provides a descriptor corresponding to the field1206 that indicates the unspecialized field type 1208 for the field1206. The unspecialized containing type 1202 may further define a method(not illustrated) associated with an unspecialized method parameter typeand an unspecialized method return value type. In particular, theunspecialized containing type 1202 provides a descriptor correspondingto the method that indicates the unspecialized method parameter type andthe unspecialized method return value type for the method.

A caller may call the unspecialized containing type 1202, the field 1208in the unspecialized containing type 1202, and/or the method in theunspecialized containing type 1202. The caller may make the call withina particular context 1204. The caller may specify the context 1204 by,for example, specifying an anchor value.

The unspecialized containing type 1202 specialized within the particularcontext 1204 is a specialized containing type 1214. The specializedcontaining type 1214 may be the unspecialized containing type 1202called in conjunction with the anchor value. An object (or class mirror)represents the unspecialized containing type 1202; a different object(or class mirror) represents the specialized containing type 1214.

Additionally or alternatively, the unspecialized field type 1208specialized within the particular context 1204 is a specialized fieldtype 1212. The specialized field type 1212 is the unspecialized fieldtype 1208 adorned with one or more type restrictions 1210 indicated bythe particular context 1204. The type restrictions 1210 may be indicatedby the anchor value. A type restriction 1210 prohibits the field 1206from assuming one or more values that are valid for the unspecializedfield type 1208.

Additionally or alternatively, the unspecialized method parameter typespecialized within the particular context 1204 is a specialized methodparameter type. The unspecialized method return value type specializedwithin the particular context 1204 is a specialized method return valuetype. The specialized method parameter type and/or method return valuetype is the unspecialized method parameter type and/or method returnvalue type adorned with one or more type restrictions indicated by theparticular context 1204. The type restrictions may be indicated by theanchor value. A type restriction prohibits the method parameter and/ormethod return value from assuming one or more values that are valid forthe unspecialized method parameter type and/or method return value type.A particular context 1204 may provide same or different typerestrictions to fields, method parameters, and/or method return valuesin a same specialized containing type 1214.

Therefore, an unspecialized containing type 1202 “indicates” anunspecialized field type 1208 of a field 1206; a specialized containingtype 1214 “indicates” a specialized field type 1212 of the same field1206. Additionally or alternatively, a descriptor corresponding to afield 1206 “indicates” an unspecialized field type 1208 of the field1206; a context 1204 (which may be provided by an anchor value),together with the descriptor, “indicates” a specialized field type 1212of the field 1206.

FIG. 12B illustrates an object of a specialized type according to anembodiment.

In one or more embodiments, a type (unspecialized or specialized) isassociated with a memory layout. A memory layout defines a sequence offields of the type, and a respective size of each field. Further, thetype is associated with a memory size. The memory size includes a totalsize of all fields of the type.

In one or more embodiments, an underlying unspecialized type defines (orinherits) fields 1226 a -c. A specialized type results from theunderlying unspecialized type specialized based on an anchor value 1230.In an embodiment, a memory layout 1234 for the specialized type may besame as or different from an underlying unspecialized type. Inparticular, sizes of fields 1226 a -c may be modified based on one ormore type restrictions, or other information, indicated by the anchorvalue 1230. Additionally or alternatively, a sequence of fields 1226 a-c may be modified based on one or more type restrictions, or otherinformation, indicated by the anchor value 1230. Given a modified memorylayout 1234, the specialized type may be associated with a differentmemory size than the underlying unspecialized type.

In one or more embodiments, an object of a specialized type is stored inaccordance with a memory layout and/or memory size associated with thespecialized type. As illustrated, for example, an object of aspecialized type occupies a memory space 1224 within a heap 1222. Theobject is associated with a memory layout 1234 of the specialized type.The memory layout 1234 provides how fields 1226 a -c are stored withinthe memory space 1224. The object is associated with a a memory size ofthe specialized type.

In one or more embodiments, an object of a specialized type is stored inassociation with the specialized type. As illustrated, for example, anobject of a specialized type is associated with metadata 1228. Themetadata 1228 may be stored, for example, in a header of the object. Themetadata 1228 indicates an anchor value 1230 providing specializationfor the specialized type. The metadata 1228 indicates a recordedspecialized type 1232 for the object. The recorded specialized type 1232is the specialized type of the object, as recorded in association withthe object. The recorded specialized type 1232 may be indicated, forexample, using a reference to an object (such as a class mirror)representing the specialized type.

FIG. 13 illustrates a resolved value, namely an anchor value, for aresolution state of a specialization anchor constant according to anembodiment.

In one or more embodiments, an anchor value 1302 stores a set ofspecialization decisions for an API point defined by a caller based on aselector value provided by a callee. An anchor value 1302 is an objectstored on a heap. An anchor value 1302 is an object of a particulartype, which may be referred to as “specialization anchor.” The“specialization anchor” type may define one or more fields indicatingthe following information: anchor constant 1304, selector value 1306,dependent constant 1307, memory layout 1308, memory size 1310, object(such as a class mirror) 1312 representing a specialized class, typerestriction 1314, and/or associated default anchor value 1316.

In one or more embodiments, an anchor value 1302 stores a reference toan anchor constant 1304 bound to the anchor value 1302. The referencepoints to the anchor constant 1304 within the runtime constant pool ofthe type that defines the anchor constant 1304.

In one or more embodiments, an anchor value 1302 stores reference(s) toone or more relevant selector values 1306. A particular selector value1306 is relevant to an anchor value 1302 if an anchor constant 1304 isresolved to the anchor value 1302 based on the particular selector value1306.

In one or more embodiments, an anchor value 1302 stores reference(s) toone or more dependent constants 1307. In an embodiment, a constant A isreferred to as being “dependent” on another constant B if and only if atleast one of the following circumstances is true:

-   A contains an index referring to B;-   A contains an index referring to a bootstrap method in the type that    defines A, and one of the static arguments of the bootstrap method    refers to B;-   A is an anchor constant of the “method and class parametricity” kind    and B is the corresponding anchor constant of the “class    parametricity” kind.

When an anchor value 1302 is determined, the dependent constants 1307are prepared but not necessarily resolved. Each dependent constant 1307may be independently resolved upon initial use.

In one or more embodiments, an anchor value 1302 stores a memory layout1308 for a specialized type resulting from specializing an underlyingunspecialized type based on the anchor value 1302. As an example, amemory layout 1308 for a specialized type may indicate that a field isof a particular size, whereas a memory layout for the underlyingunspecialized type may indicate that the field is of a different size.As another example, a memory layout 1308 for a specialized type mayindicate that a field is inlined, whereas a memory layout for theunderlying unspecialized type may indicate that the field is referenced.

In an embodiment, a memory layout 1308 for a specialized type may bedetermined based on one or more type restrictions 1314 applicable to oneor more API points in the specialized type. The memory layout 1308 andthe associated type restrictions 1314 may be indicated in the sameanchor value 1302.

In one or more embodiments, an anchor value 1302 stores a memory size1310 for a specialized type resulting from specializing an underlyingunspecialized type based on the anchor value 1302. The memory size 1310may be determined based on adding the respective memory sizes of thefields in the specialized type, as provided by the memory layout 1308.

In one or more embodiments, an anchor value 1302 stores a reference toan object (such as a class mirror) representing a specialized typeresulting from specializing an underlying unspecialized type based onthe anchor value 1302.

In one or more embodiments, an anchor value 1302 stores one or more typerestrictions applicable to one or more API points that are co-dependenton the anchor constant 1304. A type restriction prohibits an API pointfrom assuming a value that is valid for an unspecialized type of the APIpoint. The unspecialized type of the API point adorned with the typerestriction is a specialized type of the API point.

In one or more embodiments, an anchor value 1302 stores a reference toan associated default anchor value 1316. As described above, a defaultanchor value is resolved for an anchor constant during an initialpreparation and resolution of a type defining the anchor constant. Adefault anchor value for a particular anchor constant is referred to asbeing “associated” with an anchor value 1302 (determined in a differentcontext) for the same particular anchor constant.

FIG. 14 illustrates encapsulation of specialization decisions viarestrictions on loadable values of specialization linkage constantsaccording to an embodiment.

In one or more embodiments, a linkage constant 1402 wraps around an APIpoint that is a method, field, or constructor 1404. A resolved value1402 for the linkage constant 1402 indicates (a) the method, field, orconstructor 1404 and (b) an anchor value 1406 specializing the method,field, or constructor 1404. While the linkage constant 1402 is definedby the caller to the API point, the linkage constant 1402 has noloadable value 1408. Therefore, the anchor value 1406 is not accessibleto the caller.

In one or more embodiments, a linkage constant 1412 wraps around an APIpoint that is a class or interface 1414. A resolved value 1412 for thelinkage constant 1412 indicates (a) the class or interface 1414 and (b)an anchor value 1416 specializing the class or interface 1414. While thelinkage constant 1412 is defined by the caller to the API point, aloadable value 1418 of the linkage constant 1402 includes an objectrepresenting the specialized class 1420, as indicated by the anchorvalue 1416; the loadable value 1418 does not include other informationprovided by the anchor value 1416. Therefore, the full value of theanchor value 1406 is not accessible to the caller.

FIG. 15 illustrates a call to a parametric API point according to anembodiment.

In one or more embodiments, a bytecode 1500 includes an instruction 1502and an operand 1504.

In one or more embodiments, an instruction 1502 is associated with acommand 1511 and an instruction type 1512. A command 1511 provides anaction to be executed (such as invoking a method, creating an object,accessing a field, and/or accessing a local variable). An instructiontype 1512 provides a type of an operand 1504 (such as int, double,reference) to be operated upon using the command 1511.

In an embodiment, an instruction type 1512 of an instruction 1502 isdetermined based on a context 1510 associated with the instruction 1502.In particular an instruction type 1512 may be narrowed based on a typerestriction applicable within the context 1510. A compiler may translatea source code instruction to a virtual machine instruction of a narrowerinstruction type based on a type restriction; additionally oralternatively, an interpreter may translate a virtual machineinstruction to a machine code instruction of a narrower instruction typebased on a type restriction. As an example, an unspecialized field typeof a particular field may be Object. An instruction operating on theparticular field without specialization may be of an instruction typeassociated with reference, such as al oad. A specialized containing typemay adorn the unspecialized field type with a type restriction,restricting the particular field to values of int. The specializedcontaining type may be associated with a memory layout that inlines theint value for the particular field within an instance of the specializedtype. Hence, an instruction operating on the particular field may treatthe particular field as having a specialized field type of int. Theinstruction may be of an instruction type associated with int, such asiload.

In one or more embodiments, an operand 1504 may be an API point 708, alinkage constant 1506, and/or another data structure. Where the operand1504 is a linkage constant 1506, the operand 1504 adds parametricinformation to the API point 1508 that is wrapped by the linkageconstant 1506 and being called. The linkage constant 1506 furthersupplies a context 1510 for the call to the API point 1508. The context1510 may indicate one or more type restrictions to be applied to a type,field, method parameter, and/or method return value.

In an embodiment, a context 1510 within which a same resolution state ofan anchor constant is accessed may be defined by a selector value and/oran anchor value. Where the linkage constant 1506 is not resolved, acontext 1510 is provided as a selector value. Every access to an anchorconstant using a same selector value accesses the same resolution statefor the anchor constant. (An access to the anchor constant using adifferent selector value may access a different resolution state for theanchor constant.) Where the linkage constant 1506 is resolved, a context1510 is provided as a selector value and/or an anchor value. Everyaccess to an anchor constant using a same anchor value accesses the sameresolution state for the anchor constant. (An access to the anchorconstant using a different anchor value may access a differentresolution state for the anchor constant.)

5. Resolving a Specialization Anchor Constant and a SpecializationLinkage Constant

One or more operations illustrated in FIGS. 16-19 may be modified,rearranged, or omitted all together. Accordingly, the particularsequence of operations illustrated in FIGS. 16-19 should not beconstrued as limiting the scope of one or more embodiments.

FIG. 16 illustrates an example set of operations for preparing andresolving a type in a runtime environment, including preparing andresolving a parametric constant within a default context, according toan embodiment. The operations of FIG. 16 may be performed within alinking stage. Linking a type in a runtime environment involvesverifying and preparing the type, the type's direct superclass, thetype's direct superinterfaces, and the type's element type (if the typeis an array type), if necessary. Linking also involves resolution ofsymbolic references in the type, though not necessarily at the same timeas the type is verified and prepared. Further examples relating tolinking be found in the Java® Virtual Machine Specification, Java SE 16Edition, published Feb. 12, 2021, available athttps://docs.oracle.com/javase/specs/jvms/se16/html/index.html, which ishereby incorporated by reference in full.

One or more embodiments include verifying a type that depends on ananchor constant (Operation 1604). Verification ensures that the binaryrepresentation of a type is structurally correct. Verification may causeadditional types to be loaded but need not cause the additional types tobe verified or prepared.

One or more embodiments include preparing the type (Operation 1606).Preparation involves creating the static fields for the type. Preparingfurther involves initializing the static fields to the static field'sdefault values. Preparation does not necessarily require the executionof any virtual machine code; explicit initializers for static fields maybe executed as part of initialization, not preparation. Duringpreparation of the type, loading constraints may be imposed. Preparationof the type may occur at any time following creation of the type but iscompleted prior to initialization of the type.

In one or more embodiments, preparing the type includes preparing theanchor constant (Operation 1608). Preparing the anchor constant includesallocating memory space for a default resolution state of the anchorconstant (Operation 1610). The virtual machine allocates a memory spacein heap for storing a (default) resolved value for the defaultresolution state of the anchor constant. The size of the memory spaceallocated is determined based on a size associated with a type of theresolved value for the anchor constant. In an embodiment, the resolvedvalue is an anchor value, which an object of a “specialization anchor”type. Therefore, a memory space of a memory size associated with the“specialization anchor” type is allocated.

One or more embodiments include resolving the type (Operation 1612).Many virtual machine instructions rely on symbolic references in aruntime constant pool. The symbolic reference may be to, for example,(i) a class or interface, (ii) a field, (iii) a method, (iv) a methodtype, (v) a method handle, or (vi) a dynamically-computed constant.Initially, all symbolic references in the runtime constant pool areunresolved. Execution of instructions relying on a symbolic referencerequires resolution of the symbolic reference. Resolution is the processof dynamically determining one or more concrete values from a symbolicreference in the runtime constant pool. If a resolution succeeds,subsequent attempts to resolve the symbolic reference succeed triviallyand result in the same entity produced by the initial resolution. If thesymbolic reference is to a dynamically-computed constant, the bootstrapmethod is not necessarily re- executed for the subsequent attempts. Ifthe resolution does not succeed, subsequent attempts to resolve thesymbolic reference fail with the same error that was thrown as a resultof the initial resolution attempt. If the symbolic reference is to adynamically-computed constant, the bootstrap method is not necessarilyre-executed for the subsequent attempts.

In one or more embodiments, resolving the type includes resolving theanchor constant within a default context (Operation 1614). Resolving theanchor constant within the default context includes obtaining a defaultresolved value (or error) within the default context (Operation 1616).The default context applies where a caller attempts to use the anchorconstant (for example, in association with an API point or constant thatdepends on the anchor constant) without supplying an explicit context.If the anchor constant is successfully resolved within the defaultcontext, then a default resolved value is determined. The defaultresolved value is a default anchor value for the anchor constant. Thedefault anchor value may apply no modifications to an unspecialized typeof the API point. For example, the default anchor value may indicate amemory layout and/or memory size that is same as the unspecialized typeof the API point; the default anchor value may impose no typerestrictions on the API point. If the anchor constant is notsuccessfully resolved within the default context, then an error withinthe default context is determined. Further examples relating toobtaining a resolved value (or error) within a particular context aredescribed below with reference to FIG. 19.

In an embodiment, the resolution of the anchor constant within thedefault context is performed during a resolution of the type definingthe anchor constant. Therefore, the resolution process may be performedvia executing a method defined by the runtime environment, rather than abootstrap method corresponding to the anchor constant.

One or more embodiments include storing the default resolved value as apermanent value for the default resolution state of the anchor constant(Operation 1616). If the anchor constant is successfully resolved withinthe default context, then the default resolved value is stored as apermanent value for the default resolution state of the anchor constant.If the anchor constant is not successfully resolved within the defaultcontext, then the error within the default context is stored as apermanent value for the default resolution state of the anchor constant.

FIG. 17 illustrates an example set of operations for executing aparametric API point within a particular context, including selecting aresolution state of a parametric constant associated with the particularcontext, according to an embodiment.

One or more embodiments include receiving a call to an API point thatdepends on an anchor constant (Operation 1702). A callee receives a callto an API point that depends on an anchor constant. The API point isdefined in the callee, or the API point is defined in a supertype of thecallee and inherited by the callee. The anchor constant is defined inthe callee, such as being defined within a constant pool table of thecallee.

One or more embodiments include determining whether the call isassociated with a linkage constant providing a particular context forexecution of the API point (Operation 1704). The callee determineswhether the call is associated with a linkage constant providing aparticular context for execution of the API point. Where a call isassociated with a linkage constant, the linkage constant is passed froma call stack associated with the caller to a call stack associated withthe callee. Therefore, the call is associated with a linkage constant ifthe linkage constant is accessible within the call stack associated withthe callee.

If the call is not associated with a linkage constant, one or moreembodiments include selecting a default resolution state of the anchorconstant (Operation 1716). The callee selects a default resolution stateof the anchor constant. The default resolution state may be determinedduring a linking stage of a type defining the anchor constant, asdescribed above with reference to FIG. 16.

If the call is associated with a linkage constant, one or moreembodiments include determining whether the particular context matchesany existing contexts associated with resolution states of the anchorconstant (Operation 1718). Different resolution states of the anchorconstant are associated with different contexts. The callee determineswhether the particular context provided by the call matches any existingcontexts associated with resolution states of the anchor constant.

If the particular context is provided as an unresolved linkage constant,and the unresolved linkage constant is not associated with a selectorvalue that equals any anchor value previously resolved for the anchorconstant, then the callee determines that particular context does notmatch any existing contexts associated with resolution states of theanchor constant.

If the particular context is provided as an unresolved linkage constant,and the unresolved linkage constant is associated with a selector valuethat equals a particular anchor value previously resolved for the anchorconstant, then the callee determines that particular context matches acontext associated with the particular anchor value.

If the particular context is provided as a resolved linkage constant,and the resolved linkage constant is associated with a particular anchorvalue previously resolved for the anchor constant, then the calleedetermines that particular context matches a context associated with theparticular anchor value.

If the particular context matches an existing context associated with aparticular resolution state of the anchor constant, one or moreembodiments include selecting the resolution state associated with thematching context (Operation 1718). The callee selects the resolutionstate, for the anchor constant, associated with the matching context. Aresolved value for the resolution state associated with the matchingcontext thereby provides a specialization for the API point.

Hence, if (a) the particular context is provided as an unresolvedlinkage constant, and the unresolved linkage constant is associated witha selector value that equals a particular anchor value previouslyresolved for the anchor constant, or (b) the particular context isprovided as a resolved linkage constant, and the resolved linkageconstant is associated with a particular anchor value previouslyresolved for the anchor constant, then the callee selects the resolutionstate associated with the particular anchor value.

If the particular context does not match any existing context associatedwith resolution states of the anchor constant, one or more embodimentsinclude resolving the linkage constant (Operation 1708).

In one or more embodiments, resolving the linkage constant includesresolving the API point, if not yet resolved (Operation 1710). Anunresolved linkage constant initially includes a symbolic reference toan API point to be called. The symbolic reference is resolved to adirect reference to the API point.

In one or more embodiments, resolving the linkage constant includespreparing and resolving the anchor constant within the particularcontext (Operation 1712). Further examples relating to preparing andresolving an anchor constant within a particular context are describedbelow with reference to FIG. 18.

One or more embodiments include selecting the newly determinedresolution state of the anchor constant (Operation 1714). The calleeselects the newly determined resolution state of the anchor constant. Aresolved value for the newly determined resolution state therebyprovides a specialization for the API point.

One or more embodiments include executing the API point using theselected resolution state of the anchor constant (Operation 1720). Thecallee executes the API point using the selected resolution state of theanchor constant. Further examples relating to executing an API pointusing a particular resolution state of an anchor constant are describedbelow with reference to FIGS. 20-22.

One or more embodiments include executing the operations of FIG. 17 withrespect to each call to an API point that depends on the same anchorconstant. Different calls to the same API point that depends on theanchor constant trigger execution of the operations of FIG. 17, whereindifferent resolution states of the same anchor constant may be selectedbased on Operations 1714-1718. Additionally or alternatively, differentcalls to different API points that depend on the same anchor constanttrigger execution of the operations of FIG. 17, wherein differentresolution states of the same anchor constant may be selected based onOperations 1714-1718.

FIG. 18 illustrates an example set of operations for preparing andresolving a parametric constant within particular contexts provided bycalls to parametric API points dependent on the parametric constantaccording to an embodiment.

One or more embodiments include receiving a request to prepare andresolve an anchor constant within a particular context (Operation 1802).A caller makes a call to an API point that depends on an anchorconstant. The call is made within a particular context. As an example, acall to an API point is made using an instruction and an operand. Theoperand is a linkage constant. The linkage constant is unresolved. Aninformation structure for the linkage constant indicates a selectorvalue. The selector value thereby provides a particular context for thecall to the API point.

Responsive to the call to the API point, a stack frame associated withthe callee is pushed onto a call stack. The stack frame associated withthe callee corresponds to execution of the API point responsive to thecall made by the caller. Information representing the API point, such asdirect reference to the API point, is passed from the stack frameassociated with the caller to the stack frame associated with thecallee. Further information representing the particular context, such asa selector value and/or an anchor value, is passed from the stack frameassociated with the caller to the stack frame associated with thecallee. The callee determines whether the API point is associated with aparametric attribute. The callee identifies an anchor constantreferenced by the parametric attribute of the API point. The calleethereby receives a request to prepare and resolve the anchor constantwithin the particular context.

One or more embodiments include preparing the anchor constant within theparticular context (Operation 1804). Preparing the anchor constantwithin the particular context includes generating or identifying aparametric runtime constant pool corresponding to the particular context(Operation 1806). The callee identifies a set of zero or more parametricruntime constant pools associated with a type defining (or inheriting)the API point being called. The callee determines whether the particularcontext matches any context associated with the existing set ofparametric runtime constant pools. The callee identifies the parametricruntime constant pool associated with the matching context. Otherwise,the callee generates a new parametric runtime constant pool associatedwith the particular context.

Preparing the anchor constant within the particular context includesallocating a memory space, of the identified parametric runtime constantpool, for a new resolution state of the anchor constant (Operation1808). The callee allocates a memory space for a new resolution state ofthe anchor constant. The identified memory space is within theparametric runtime constant pool generated or identified at Operation1806. Examples of operations relating to allocating memory space for aresolution state of an anchor constant are further described above withreference to Operation 1610 of FIG. 16.

One or more embodiments include resolving the anchor constant within theparticular context (Operation 1810). Resolving the anchor constantwithin the particular context includes executing a bootstrap methodassociated with the anchor constant to obtain a resolved value (orerror) within the particular context (Operation 1812). The calleeidentifies an information structure for the anchor constant. Theinformation structure for the anchor constant references a bootstrapmethod corresponding to the anchor constant. The callee inputsinformation indicating the particular context into the bootstrap method.

If the anchor constant is successfully resolved within the particularcontext, then a resolved value within the particular context isdetermined. The resolved value is an anchor value for the anchorconstant within the particular context. The anchor value records a setof specialization decisions for the API point within the particularcontext. If the anchor constant is not successfully resolved within theparticular context, then an error within the particular context isdetermined. Further examples relating to obtaining a resolved value (orerror) within a particular context are described below with reference toFIG. 19.

In an embodiment, resolution of the anchor constant within a particularcontext provided by a caller may be performed via executing a bootstrapmethod corresponding to the anchor constant. Different calls to one ormore API points dependent on the same anchor constant may be made withindifferent contexts. Each call within a different context triggersexecution of the same bootstrap method corresponding to the anchorconstant. However, each execution of the bootstrap method involves theinput of different contexts, such as via different selector values.

One or more embodiments include storing the resolved value as apermanent value for the resolution state of the anchor constant withinthe particular context (Operation 1814). If the anchor constant issuccessfully resolved within the particular context, then the resolvedvalue is stored as a permanent value for the resolution state of theanchor constant within the particular context. If the anchor constant isnot successfully resolved within the particular context, then the errorwithin the particular context is stored as a permanent value for theresolution state of the anchor constant within the particular context.

FIG. 19 illustrates an example set of operations for determining ananchor value for a resolution state of an anchor constant according toan embodiment.

One or more embodiments include receiving a request to obtain an anchorvalue for an anchor constant within a context of a proposed selectorvalue (or a default context) (Operation 1902). A callee receives arequest to obtain an anchor value for an anchor constant within acontext of a proposed selector value. The proposed selector valueindicates a caller's proposed specializations for an API point dependenton the anchor constant. If no explicit context is supplied by a caller,then a default context serves as the context in which the anchor valueis to be obtained.

One or more embodiments include determining whether the proposedselector value valid with respect to the anchor constant (Operation1904). The callee executes a bootstrap method corresponding to theanchor constant. The bootstrap method includes operations configured todetermine whether the proposed selector value is valid. The proposedselector value is not valid if the proposed selector value cannot beused to generate an anchor value for the anchor constant.

If the proposed selector value is not valid, one or more embodimentsinclude generating an error (Operation 1906). The callee generates anerror. The error is recorded as a permanent value for a resolution stateof the anchor constant within the context of the proposed selectorvalue.

One or more embodiments include determining one or more typerestrictions (Operation 1908). The callee determines one or more typerestrictions associated with the API point. The callee storesinformation indicating the type restrictions within the anchor value.

As an example, a Box class may declare a field named point andparameterized over a parameter type T. During compilation, the parametertype T is erased to an unspecialized type Object. Hence, a class filefor Box indicates that the field point is of an unspecialized typeObject. The class file for Box further indicates that the field point isdependent on an anchor constant. A caller to the field point may proposeto specialize the type of point to int. The Box class may execute abootstrap method corresponding to the anchor constant. Execution ofoperations of the bootstrap method may determine that a type restrictionis to be applied on the field point within the context provided by thecaller's specialization proposal. The type restriction may restrict thevalue of the field point to values conforming to the int type.

One or more embodiments include determining a memory layout (Operation1910). The callee determines a memory layout associated with aspecialized type of the API point. The memory layout may be determinedbased on (a) the fields in the unspecialized type, and (b) one or moretype restrictions, determined at Operation 1908, on the fields in theunspecialized type. A type restriction may be used to optimize thememory layout for the specialized type, as compared with theunspecialized type. The memory layout for the specialized type includesan entry for each field that is in the unspecialized type (via directdefinition by the unspecialized type or inheritance by the unspecializedtype)—even though the specialized type itself does not define thefields. The callee stores information indicating the memory layoutwithin the anchor value.

Continuing the above example, an int value may be associated with a sizeof 4 bytes; a reference value may be associated with a size of 8 bytes.The unspecialized type of Object associated with the field pointprovides that a value for the field stores a reference to an object. Thereference is 8 bytes. The specialized type, which is the Object typerestricted to values of int, associated with the field point providesthat a value for the field stores an int value. The int value requires 4bytes. Therefore, the callee may reduce a memory size allocated for thefield point from 8 bytes to 4 bytes. The callee may further determine toinline the int value within an instance of the specialized type. Thememory layout is therefore optimized based on the type restrictionrestricting to values of int.

One or more embodiments include determining a memory size (Operation1912). The callee determines a memory size associated with a specializedtype of the API point. The memory size may be determined based on amemory layout determined at Operation 1910. The callee storesinformation indicating the memory size within the anchor value.

Continuing the above example, a memory size for the field point has beenreduced from 8 bytes to 4 bytes. Accordingly, the callee determines thatan overall memory size associated with the specialized type Box<Int> is4 bytes less than an overall memory size associated with theunspecialized type Box.

One or more embodiments include generating an object representing aspecialized type (Operation 1914). The callee generates an object (suchas a class mirror) representing the specialized type resulting from theunspecialized type of the API point specialized within the context ofthe proposed selector value. The callee stores a reference to the objectrepresenting the specialized type within the anchor value.

One or more embodiments include identifying the associated anchorconstant, selector value, and default anchor value (Operation 1916). Thecallee identifies an anchor constant associated with the anchorvalue—that is, the anchor constant on which the API point being calleddepends. The callee identifies the selector value associated with theanchor value—that is, the selector value serving as an input to abootstrap method of the anchor constant that obtains the anchor value.The callee identifies the default anchor value associated with theanchor value—that is, the anchor value determined in the default contextfor the same anchor constant. The callee stores references to theassociated anchor constant, selector value, and default anchor valuewithin the anchor value.

One or more embodiments include returning the anchor value obtainedwithin the context of the proposed selector value (Operation 1918). Thecallee returns the anchor value obtained within the context of theproposed selector value.

6. Executing Parametric API Points within Particular Contexts

One or more operations illustrated in FIGS. 20-22 may be modified,rearranged, or omitted all together. Accordingly, the particularsequence of operations illustrated in FIGS. 20-22 should not beconstrued as limiting the scope of one or more embodiments.

FIG. 20 illustrates an example set of operations for executing aninvocation instruction to invoke a parametric method according to anembodiment.

One or more embodiments include identifying an invocation instruction toinvoke a parametric callee method within a particular context (or adefault context) (Operation 2002). A caller provides an invocationinstruction and one or more operands. The one or more operands indicate(a) a callee method to be invoked and (b) a particular context withinwhich to invoke the callee method. In the embodiment, an operandassociated with the invocation instruction is a linkage constant. Aresolved value for the linkage constant includes a reference to thecallee method, and a reference to an anchor value associated with theparticular context.

In an embodiment, operands of the invocation instruction do notexplicitly provide any context. If no context is explicitly provided,then a default context applies.

One or more embodiments include pushing a stack frame, onto a callstack, corresponding to execution of the callee method responsive to theinvocation instruction (Operation 2004). A stack frame corresponding toexecution of the callee method responsive to the invocation instructionis pushed onto the call stack. The stack frame corresponding toexecution of the callee method is now the active frame.

One or more embodiments include passing the particular context to theactive stack frame (Operation 2006). Information indicating theparticular context within which the callee method is to be invoked ispassed from the stack frame associated with the caller to the stackframe associated with the callee. The information indicating theparticular context may be, for example, an anchor value.

One or more embodiments include storing a constant pool reference,corresponding to the active stack frame, that indicates an invariantruntime constant pool and a parametric runtime constant pool associatedwith the particular context (Operation 2008). A virtual machinedetermines that the particular context for invoking the callee methodhas been provided to the active frame. The virtual machine determineswhether the callee method is associated with an anchor constant. Thevirtual machine determines, for example, whether the callee method isassociated with a parametric attribute that references an anchorconstant. If the callee method is associated with an anchor constant,then the callee method is a “parametric callee method.” The virtualmachine identifies a set of one or more parametric runtime constantpools that (a) are associated with a type that defines (or inherits) thecallee method and (b) store resolution states of the anchor constant.The virtual machine identifies a respective context associated with eachof the set of parametric runtime constant pools. The virtual machinecompares the particular context provided by the caller and the contextsassociated with the set of parametric runtime constant pools to find amatch. The virtual machine identifies one of the set of parametricruntime constant pools associated with a context matching the particularcontext. The virtual machine also identifies an invariant runtimeconstant pool associated with the type that defines (or inherits) thecallee method.

The virtual machine stores as a constant pool reference, correspondingto the active stack frame, information indicating the identifiedinvariant runtime constant pool and the identified parametric runtimeconstant pool associated with the particular context.

One or more embodiments include passing zero or more arguments to theactive stack frame (Operation 2010). If the callee method is notassociated with any method parameters, no arguments are passed to theactive stack. Otherwise, one or more arguments are passed from the stackframe associated with the caller to the stack frame associated with thecallee.

In an embodiment, passing the argument includes determining whether theargument is to be inlined or referenced in the callee stack frame, anddetermining a memory size to be allocated for the inlined value or thereference in the callee stack frame. The virtual machine determineswhether the particular context for invoking the callee method isassociated with a specialized type for any method parameter. As anexample, an anchor value may specify a type restriction on a methodparameter. The type restriction may limit a memory size needed for themethod parameter. Additionally or alternatively, the anchor value mayspecify whether the method parameter is to be inlined or referenced,and/or a memory size of the method parameter. Based on the specializedtype of a method parameter within the particular context, the virtualmachine determines whether the argument is to be inlined or referencedin the callee stack frame, and determines a memory size to be allocatedfor the inlined value or the reference in the callee stack frame. Basedon the memory size to be allocated, the virtual machine allocates memoryspace for the argument as a local variable in the callee stack frame.Also based on the memory size to be allocated, the virtual machineallocates memory space for the argument on an operand stack of thecallee frame.

One or more embodiments include executing the parametric callee methodwithin the particular context (Operation 2011). The virtual machineexecutes the parametric callee method within the particular context.

One or more embodiments include accessing resolution states of one ormore constants from the invariant runtime constant pool and/orparametric runtime constant pool (Operation 2012). Executing theparametric callee method may include accessing resolution states of oneor more constants defined by the type defining (or inheriting) theparametric callee method. The constants are accessed based on theconstant pool reference of the active frame. Since the constant poolreference points to (a) the invariant runtime constant pool associatedwith the type and (b) the parametric runtime constant pool associatedwith the type and associated with the particular context, resolutionstates of invariant constants are accessed from the invariant runtimeconstant pool associated with the type, and resolution states ofparametric constants (for example, the anchor constant and constantsdependent on the anchor constant) are accessed from the parametricruntime constant pool associated with the type and associated with theparticular context.

One or more embodiments include identifying a type restriction appliedto the method parameter and/or method return value within the particularcontext (Operation 2014). The virtual machine identifies a typerestriction applied to the method parameter and/or method return valuewithin the particular context. The virtual machine identifies the anchorvalue associated with the particular context. The anchor value specifiesany type restrictions applied to any method parameters and/or methodreturn value of the callee method.

One or more embodiments include type checking and/or casting an argumentand/or method return value based on the type restriction (Operation2016). The virtual machine type checks an argument and/or method returnvalue based on the type restriction. The virtual machine determineswhether the argument for the callee method conforms to a typerestriction. Additionally or alternatively, the virtual machinedetermines whether the method return value for the callee methodconforms to a type restriction. As an example, an unspecialized type ofa method parameter of a callee method is Object, and a type restrictionrestricts the method parameter to values of int. An argument to thecallee method may be a String such as “hello.” The virtual machine typechecks the argument based on the unspecialized type of the methodparameter. The virtual machine determines that the “hello” Stringconforms to the Object type. Additionally the virtual machine typechecks the argument based on the type restriction. The virtual machinedetermines that the “hello” String does not conform the int type. Thevirtual machine may determine that the argument does not pass typechecking.

If an argument and/or method return value does not pass type checking,the virtual machine may either generate an error, or cast the argumentand/or method return value to conform to the type restriction. As anexample, an unspecialized type of a method parameter of a callee methodis Vehicle, and a type restriction restricts the method parameter tovalues of Bicycle. Bicycle is a sub-class of Vehicle, and adds a fieldgears. An argument to the callee method may be a Vehicle object. Thevirtual machine casts the Vehicle object to conform to the Bicycle type.In particular, the virtual machine may add the field gears andinitialize the field to a default value. As used herein, the term “cast”may also include “convert.” As another example, an unspecialized type ofa method parameter of a callee method is Vehicle, and a type restrictionrestricts the method parameter to values of int. int is not a sub-classof Vehicle. An argument to the callee method may be a Vehicle object.The virtual machine converts the Vehicle object to conform to the inttype. In particular, the virtual machine may apply a conversion rule tothe Vehicle object to convert the Vehicle object into an int value.

Since different type restrictions may be applicable within differentcontexts, the same callee method executed within different contexts mayinvolve type checking, converting, and/or casting based on differenttype restrictions. Whereas a certain argument to a callee method mayconform with a type restriction within a particular context, the sameargument to the same callee method may not conform with another typerestriction within another context.

One or more embodiments include determining whether to inline theargument and/or method return value based on the type restriction(Operation 2018). The virtual machine determines whether to inline theargument and/or method return value based on the type restriction. Ifthe type restriction restricts the method parameter and/or method returnvalue to a primitive type, then the virtual machine may determine toline the argument and/or method return value.

Since different type restrictions may be applicable within differentcontexts, a callee method executed within a particular context mayinvolve inlining, while the same callee method executed within anothercontext may involve using a reference.

One or more embodiments include inlining the argument and/or methodreturn value onto an operand stack corresponding to the active stackframe (Operation 2020). The virtual machine inlines the argument and/ormethod return value onto an operand stack corresponding to the activestack frame. The virtual machine stores the data value of the argumentand/or method return value directly onto the operand stack, rather thanstoring a reference to the argument and/or method return value.

One or more embodiments include pushing a reference to the argumentand/or method return value onto the operand stack (Operation 2022). Thevirtual machine stores references to the argument and/or method returnvalue onto the operand stack corresponding to the active stack frame.

One or more embodiments include passing the method return value to thestack frame associated with the caller (Operation 2024). The virtualmachine passes the method return value to the caller stack frame. If themethod return value is inlined, the virtual machine determines a memorysize associated with the inlined data value. If the method return valueis referenced, the virtual machine determines a memory size associatedwith the reference. The virtual machine allocates a memory space, basedon the determined memory size, on an operand stack of the caller stackframe for the method return value. The virtual machine stores the methodreturn value into the allocated memory space.

FIG. 21 illustrates an example set of operations for executing acreation instruction to create an instance of a parametric typeaccording to an embodiment.

One or more embodiments include identifying a creation instruction tocreate an object based on an unspecialized type within a particularcontext (or a default context), wherein the object is of a specializedtype that is the unspecialized type within the particular context(Operation 2102). A caller provides a creation instruction and one ormore operands. The one or more operands indicate (a) an unspecializedtype and (b) a particular context applicable to the unspecialized type.The object to be created would be of a specialized type, which is theunspecialized type within the particular context. In an embodiment, anoperand associated with the creation instruction is a linkage constant.A resolved value for the linkage constant includes a reference to theunspecialized type, and a reference to an anchor value associated withthe particular context. The specialized type of the object to be createdis the unspecialized type adorned with the anchor value. In anembodiment, the anchor value includes a reference to an object (such asa class mirror) representing the specialized type.

In an embodiment, operands of the invocation instruction do notexplicitly provide any context. If no context is explicitly provided,then a default context applies. The object to be created would be of aspecialized type that is the unspecialized type within the defaultcontext. The specialized type of the object to be created is theunspecialized type adorned with a default anchor value.

One or more embodiments include identifying a memory size and/or memorylayout associated with the specialized type (Operation 2104). Thevirtual machine identifies a memory size and/or memory layout associatedwith the specialized type. The anchor value specifies the memory sizeand/or memory layout associated with the specialized type.

One or more embodiments include creating the object, includingallocating memory space to the object based on the memory size and/ormemory layout associated with the specialized type (Operation 2106). Thevirtual machine allocates memory space to the object based on the memorysize and/or memory layout associated with the specialized type.

Since different memory sizes and/or memory layouts are associated withdifferent contexts, an object created based on an unspecialized typewithin different contexts involves allocating memory space of differentmemory sizes and/or memory layouts. As an example, one object createdbased on an unspecialized type within a particular context may have aninlined value for a particular field, while another object created basedon the same unspecialized type but within another context may have areferenced value for the particular field.

One or more embodiments include identifying an invocation instruction toinvoke a constructor within the particular context (or in the defaultcontext) (Operation 2108). The caller provides an invocation instructionand one or more operands. The one or more operands indicate (a) aconstructor to be invoked, and (b) optionally the particular contextwithin which to invoke the constructor. The constructor is defined bythe unspecialized type (or by a supertype of the unspecialized type).The constructor may but does not necessarily depend on an anchorconstant. The constructor may but does not necessarily depend on thesame anchor constant on which the unspecialized type depends. If theconstructor depends on an anchor constant, then the invocationinstruction may supply the particular context for invoking theconstructor. If the constructor depends on an anchor constant and theinvocation instruction does not supply any context, then the defaultcontext is assumed.

One or more embodiments include identifying the constructor defined (orinherited) by the unspecialized type (Operation 2110). The virtualmachines identified the constructor that is either defined by theunspecialized type, or defined by a supertype of the unspecialized typeand inherited by the unspecialized type. A class file of theunspecialized type (or the supertype) includes an information structurefor the constructor, specifying a descriptor for the constructor.

One or more embodiments include identifying any type restrictionsapplied on a parameter and/or return value of the constructor within theparticular context (Operation 2112). The virtual machine executes theconstructor. If the constructor depends on an anchor constant, thevirtual machine executes the constructor within the particular contextprovided with the invocation instruction (or the default context, ifnone is provided). The virtual machine identifies any type restrictionsapplied on a parameter and/or return value of the constructor within theparticular context. Examples of operations for identifying a typerestriction on a parameter and/or return value are described above withreference to Operation 2014 of FIG. 20.

One or more embodiments include initializing the object, includingexecuting the constructor within the particular context (Operation2114). Executing the constructor within the particular context includestype checking, casting, and/or inlining parameters and/or a return valuebased on type restrictions applicable within the particular context.Examples of operations relating to type checking, casting, and inliningare described above with reference to Operations 2016-2022 of FIG. 20.

One or more embodiments include storing, as metadata of the object,information indicating the specialized type of the object (Operation2116). The virtual machine stores, as metadata of the newly createdobject, information indicating the specialized type of the object. As anexample, a header of an object includes a class reference indicating theobject's class. The class reference of an object of a specialized typepoints to an object (such as a class mirror) representing thespecialized type. A reference to the object representing the specializedtype may be identified from the anchor value associated with theparticular context.

FIG. 22 illustrates an example set of operations for executing an accessinstruction to access a parametric field according to an embodiment.

One or more embodiments include identifying an access instruction toaccess a parametric field of an object within a particular context (or adefault context), wherein the object is expected to be of a particularspecialized type that is an unspecialized type within the particularcontext (Operation 2202). A caller provides an access instruction andone or more operands. The one or more operands indicate (a) a field ofan object and (b) a particular context within which to access the field.The particular context indicated by the operands, applied to someunspecialized type, results in a particular specialized type that isexpected of the object. Accessing the field may include retrieving adata value from the field and/or storing a data value to the field. Inan embodiment, an operand associated with the access instruction is alinkage constant. A resolved value for the linkage constant includes areference to the field, and a reference to an anchor value associatedwith the particular context. The anchor value specifies a classreference to an object (such as a class mirror) representing theparticular specialized type that is expected of the object.

In an embodiment, operands of the access instruction do not explicitlyprovide any context. If no context is explicitly provided, then adefault context applies.

One or more embodiments include identifying a recorded specialized typestored in association with the object (Operation 22). The virtualmachine identifies metadata of the object, such as a header of theobject. The virtual machine identifies, within the metadata, a classreference. The class reference points to a specialized type of theobject, which is referred to herein as a “recorded specialized type” ofthe object. Additionally or alternatively, the virtual machineidentifies, within the metadata, a reference to an anchor value bound tothe object. The anchor value includes a class reference that points tothe recorded specialized type of the object.

One or more embodiments include determining whether the particularspecialized type matches the recorded specialized type (Operation 2206).The virtual machine determines whether there is a match between (a) theparticular specialized type associated with the particular contextprovided at Operation 2202 and (b) the recorded specialized typedetermined at Operation 2204.

If the particular specialized type matches the recorded specializedtype, one or more embodiments include selecting the particular contextfor use (Operation 2214). The virtual machine selects the particularcontext provided at Operation 2202 for use.

If the particular specialized type does not match the recordedspecialized type, one or more embodiments include determining whether adefault context can be used (Operation 2208). The virtual machinedetermines whether a default context can be used. The default contextcan be used if the field of the object can be accessed based on a memorylayout for the unspecialized type.

If the default context cannot be used, one or more embodiments includereturning an error (Operation 2210). The virtual machine returns anerror.

If the default context can be used, one or more embodiments includeselecting the default context for use (Operation 2212). The virtualmachine selects the default context for use.

One or more embodiments include accessing the field of the object withinthe selected context (Operation 2214). The virtual machine accesses thefield of the object within the selected context.

One or more embodiments include identifying a type restriction appliedto the field and/or memory layout associated with the object within theselected context (Operation 2218). The virtual machine identifying atype restriction applied to the field and/or memory layout within theselected context. The virtual machine identifies an anchor valueassociated with the particular context, which may be (a) the anchorvalue provided with the access instruction at Operation 2202, and/or (b)the anchor value recorded in association with the object and identifiedat Operation 2204. The anchor value specifies any type restrictionsapplied to the field. The anchor value specifies a memory layout for theparticular specialized type of the object.

One or more embodiments include type checking and/or casting a value forthe field of the object based on the type restriction applied to thefield within the selected context (Operation 2220). Examples ofoperations relating to type checking and/or casting a value aredescribed above with reference to Operation 2016 of FIG. 20.

One or more embodiments include identifying a memory space for the fieldof the object based on the memory layout within the selected context(Operation 2222). The virtual machine identifies a memory space for thefield of the object based on the memory layout within the selectedcontext. As an example, a memory layout may indicate a sequence offields of a specialized type, such as name, age, address. The memorylayout may indicate a type and/or size of each field. The name field maytake 8 bytes; the age field may take 4 bytes; the address field may take8 bytes. Hence, the name field may correspond to an index of 0; the agefield may correspond to an index of 8 (index of 0 plus 8 bytes of thename field); the address field may correspond to an index of 12 (indexof 8 plus 4 bytes of the age field). A virtual machine may access afield by calculating the index corresponding to the field based on thememory layout.

One or more embodiments include determining whether to inline the valuefor the field based on the type restriction (Operation 2224). Thevirtual machine determines whether to inline the value for the fieldbased on the type restriction. If the type restriction restricts thevalue to conform to a primitive type, then the virtual machine maydetermine to line the value.

Where the access instruction is to get a field value, one or moreembodiments include inlining the value onto an operand stack (Operation2226). The virtual machine inlines the value for the field onto theoperand stack, rather than a reference to the value for the field. Thevirtual machine then processes the inlined value from the operand stack.Alternatively, where the access instruction is to put a field value, oneor more embodiments include popping an inlined value from the operandstack (operation not illustrated). The virtual machine pops the inlinedvalue for the field from the operand stack, rather than a reference tothe value for the field. The virtual machine then stores the inlinedvalue into the memory space for the object within the heap.

Where the access instruction is to get a field value, one or moreembodiments include pushing a reference to the value onto the operandstack (Operation 22). The virtual machine pushes a reference to thevalue for the field onto the operand stack. The virtual machine thenaccesses the value for the field based on the reference. Alternatively,where the access instruction is to put a field value, one or moreembodiments include popping a reference to the value from the operandstack (operation not illustrated). The virtual machine pops a referenceto the value for the field from the operand stack. The virtual machinethen stores the referenced value into the memory space for the objectwithin the heap.

7. Example Embodiments

7.1. Executing a Parametric Method Dependent on an Anchor Constant

A first embodiment includes:

while within a first stack frame, on a call stack, corresponding to afirst execution of a first caller method: identifying a first invocationinstruction to invoke a callee method within a first context; wherein afirst type restriction is applied on one or more of a method parameterand a method return value of the callee method within the first context,and the first type restriction prohibits a first value for one or moreof the method parameter and the method return value. Responsive toidentifying the first invocation instruction: pushing a second stackframe, onto the call stack, corresponding to a second execution of thecallee method responsive to the first invocation instruction; passinginformation representing the first context from the first stack frame tothe second stack frame;

while within the second stack frame, on the call stack, corresponding tothe second execution of the callee method: processing one or more of themethod parameter and the method return value based on the first typerestriction that is applied within the first context;

while within a third stack frame, on the call stack, corresponding to athird execution of a second caller method: identifying a secondinvocation instruction to invoke the callee method within a secondcontext; wherein a second type restriction is applied on one or more ofthe method parameter and the method return value within the secondcontext, the second type restriction prohibits a second value for one ormore of the method parameter and the method return value, and the firsttype restriction and the second type restriction are different;

responsive to identifying the second invocation instruction: pushing afourth stack frame, onto the call stack, corresponding to a fourthexecution of the callee method responsive to the second invocationinstruction; passing information representing the second context fromthe third stack frame to the fourth stack frame;

while within the fourth stack frame, on the call stack, corresponding tothe fourth execution of the callee method: processing one or more of themethod parameter and the method return value based on the second typerestriction that is applied within the second context.

A second embodiment includes:

identifying a plurality of runtime constant pools that are (a)associated with a type and (b) respectively correspond to a plurality ofcontexts, wherein one or more of the type and a supertype of the typedefines a callee method;

while within a first stack frame, on a call stack, corresponding to afirst execution of a first caller method: identifying a first invocationinstruction to invoke the callee method within a first context of theplurality of contexts;

responsive to the first invocation instruction: pushing a second stackframe, onto the call stack, corresponding to a second execution of thecallee method responsive to the first invocation instruction;determining that a first runtime constant pool, of the plurality ofruntime constant pools, corresponds to the first context of theplurality of contexts; storing, into the second stack frame, a firstconstant pool reference that indicates the first runtime constant pool.

7.2. Creating an Instance pf a Parametric Class Dependent on an AnchorConstant

A third embodiment includes:

identifying a first creation instruction to create a first object basedon a first unspecialized type within a first context, wherein the firstobject would be of a first specialized type that is the firstunspecialized type specialized within the first context;

creating the first object, wherein creating the first object comprises:allocating a first memory space to the first object based on one or moreof a first memory size and a first memory layout associated with thefirst specialized type;

wherein the first unspecialized type is associated with one or more of asecond memory size, different than the first memory size, and a firstmemory layout, different than the second memory layout;

initializing the first object, wherein initializing the first objectcomprises: executing a constructor defined by one or more of the firstunspecialized type and a supertype of the first unspecialized type.

7.3. Accessing a Parametric Field Dependent on an Anchor Constant

A fourth embodiment includes:

identifying a first access instruction to access a first field of afirst object within a first context, wherein the first object isexpected to be of a first specialized type, and the first specializedtype is a first unspecialized type specialized within the first context;

identifying a second specialized type stored in association with thefirst object;

determining whether the first specialized type matches the secondspecialized type;

responsive to determining that the first specialized type matches thesecond specialized type: identifying a first memory layout associatedwith one or more of the first specialized type and the secondspecialized type; wherein an unspecialized memory layout is associatedwith the first unspecialized type, and the first memory layout and theunspecialized memory layout are different; accessing the first field ofthe first object based on the first memory layout.

A fifth embodiment includes:

identifying a first access instruction of a first instruction type toaccess a first field of a first object within a first context; whereinthe first context is associated with a first type restriction on thefirst field that prohibits a first value for the first field;

executing the first access instruction to access the first field of thefirst object;

identifying a second access instruction of a second instruction type toaccess the first field of a second object within a second context;wherein the second context is associated with a second type restrictionon the first field that does not prohibit the first value for the firstfield; wherein the first instruction type and the second instructiontype are different;

executing the second access instruction to access the first field of thesecond object.

8. Miscellaneous; Extensions

Embodiments are directed to a system with one or more devices thatinclude a hardware processor and that are configured to perform any ofthe operations described herein and/or recited in any of the claimsbelow.

In an embodiment, a non-transitory computer readable storage mediumcomprises instructions which, when executed by one or more hardwareprocessors, causes performance of any of the operations described hereinand/or recited in any of the claims.

Any combination of the features and functionalities described herein maybe used in accordance with one or more embodiments. In the foregoingspecification, embodiments have been described with reference tonumerous specific details that may vary from implementation toimplementation. The specification and drawings are, accordingly, to beregarded in an illustrative rather than a restrictive sense. The soleand exclusive indicator of the scope of the invention, and what isintended by the applicants to be the scope of the invention, is theliteral and equivalent scope of the set of claims that issue from thisapplication, in the specific form in which such claims issue, includingany subsequent correction.

9. Hardware Overview

According to one embodiment, the techniques described herein areimplemented by one or more special-purpose computing devices. Thespecial-purpose computing devices may be hard-wired to perform thetechniques, or may include digital electronic devices such as one ormore application-specific integrated circuits (ASICs) or fieldprogrammable gate arrays (FPGAs) that are persistently programmed toperform the techniques, or may include one or more general purposehardware processors programmed to perform the techniques pursuant toprogram instructions in firmware, memory, other storage, or acombination. Such special-purpose computing devices may also combinecustom hard-wired logic, ASICs, or FPGAs with custom programming toaccomplish the techniques. The special-purpose computing devices may bedesktop computer systems, portable computer systems, handheld devices,networking devices or any other device that incorporates hard-wiredand/or program logic to implement the techniques.

For example, FIG. 23 is a block diagram that illustrates a computersystem 2300 upon which an embodiment of the invention may beimplemented. Computer system 2300 includes a bus 2302 or othercommunication mechanism for communicating information, and a hardwareprocessor 2304 coupled with bus 2302 for processing information.Hardware processor 2304 may be, for example, a general purposemicroprocessor.

Computer system 2300 also includes a main memory 2306, such as a randomaccess memory (RAM) or other dynamic storage device, coupled to bus 2302for storing information and instructions to be executed by processor2304. Main memory 2306 also may be used for storing temporary variablesor other intermediate information during execution of instructions to beexecuted by processor 2304. Such instructions, when stored innon-transitory storage media accessible to processor 2304, rendercomputer system 2300 into a special-purpose machine that is customizedto perform the operations specified in the instructions.

Computer system 2300 further includes a read only memory (ROM) 2308 orother static storage device coupled to bus 2302 for storing staticinformation and instructions for processor 2304. A storage device 2310,such as a magnetic disk or optical disk, is provided and coupled to bus2302 for storing information and instructions.

Computer system 2300 may be coupled via bus 2302 to a display 2312, suchas a cathode ray tube (CRT), for displaying information to a computeruser. An input device 2314, including alphanumeric and other keys, iscoupled to bus 2302 for communicating information and command selectionsto processor 2304. Another type of user input device is cursor control2316, such as a mouse, a trackball, or cursor direction keys forcommunicating direction information and command selections to processor2304 and for controlling cursor movement on display 2312. This inputdevice typically has two degrees of freedom in two axes, a first axis(e.g., x) and a second axis (e.g., y), that allows the device to specifypositions in a plane.

Computer system 2300 may implement the techniques described herein usingcustomized hard-wired logic, one or more ASICs or FPGAs, firmware and/orprogram logic which in combination with the computer system causes orprograms computer system 2300 to be a special-purpose machine. Accordingto one embodiment, the techniques herein are performed by computersystem 2300 in response to processor 2304 executing one or moresequences of one or more instructions contained in main memory 2306.Such instructions may be read into main memory 2306 from another storagemedium, such as storage device 2310. Execution of the sequences ofinstructions contained in main memory 2306 causes processor 2304 toperform the process steps described herein. In alternative embodiments,hard-wired circuitry may be used in place of or in combination withsoftware instructions.

The term “storage media” as used herein refers to any non-transitorymedia that store data and/or instructions that cause a machine tooperate in a specific fashion. Such storage media may comprisenon-volatile media and/or volatile media. Non-volatile media includes,for example, optical or magnetic disks, such as storage device 2310.Volatile media includes dynamic memory, such as main memory 2306. Commonforms of storage media include, for example, a floppy disk, a flexibledisk, hard disk, solid state drive, magnetic tape, or any other magneticdata storage medium, a CD-ROM, any other optical data storage medium,any physical medium with patterns of holes, a RAM, a PROM, and EPROM, aFLASH-EPROM, NVRAM, any other memory chip or cartridge.

Storage media is distinct from but may be used in conjunction withtransmission media. Transmission media participates in transferringinformation between storage media. For example, transmission mediaincludes coaxial cables, copper wire and fiber optics, including thewires that comprise bus 2302. Transmission media can also take the formof acoustic or light waves, such as those generated during radio-waveand infra-red data communications.

Various forms of media may be involved in carrying one or more sequencesof one or more instructions to processor 2304 for execution. Forexample, the instructions may initially be carried on a magnetic disk orsolid state drive of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 2300 canreceive the data on the telephone line and use an infra-red transmitterto convert the data to an infra-red signal. An infra-red detector canreceive the data carried in the infra-red signal and appropriatecircuitry can place the data on bus 2302. Bus 2302 carries the data tomain memory 2306, from which processor 2304 retrieves and executes theinstructions. The instructions received by main memory 2306 mayoptionally be stored on storage device 2310 either before or afterexecution by processor 2304.

Computer system 2300 also includes a communication interface 2318coupled to bus 2302. Communication interface 2318 provides a two-waydata communication coupling to a network link 2320 that is connected toa local network 2322. For example, communication interface 2318 may bean integrated services digital network (ISDN) card, cable modem,satellite modem, or a modem to provide a data communication connectionto a corresponding type of telephone line. As another example,communication interface 2318 may be a local area network (LAN) card toprovide a data communication connection to a compatible LAN. Wirelesslinks may also be implemented. In any such implementation, communicationinterface 2318 sends and receives electrical, electromagnetic or opticalsignals that carry digital data streams representing various types ofinformation.

Network link 2320 typically provides data communication through one ormore networks to other data devices. For example, network link 2320 mayprovide a connection through local network 2322 to a host computer 2324or to data equipment operated by an Internet Service Provider (ISP)2326. ISP 2326 in turn provides data communication services through theworld wide packet data communication network now commonly referred to asthe “Internet” 2328. Local network 2322 and Internet 2328 both useelectrical, electromagnetic or optical signals that carry digital datastreams. The signals through the various networks and the signals onnetwork link 2320 and through communication interface 2318, which carrythe digital data to and from computer system 2300, are example forms oftransmission media.

Computer system 2300 can send messages and receive data, includingprogram code, through the network(s), network link 2320 andcommunication interface 2318. In the Internet example, a server 2330might transmit a requested code for an application program throughInternet 2328, ISP 2326, local network 2322 and communication interface2318.

The received code may be executed by processor 2304 as it is received,and/or stored in storage device 2310, or other non-volatile storage forlater execution.

In the foregoing specification, embodiments of the invention have beendescribed with reference to numerous specific details that may vary fromimplementation to implementation. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense. The sole and exclusive indicator of the scope of the invention,and what is intended by the applicants to be the scope of the invention,is the literal and equivalent scope of the set of claims that issue fromthis application, in the specific form in which such claims issue,including any subsequent correction.

What is claimed is:
 1. One or more non-transitory computer readablemedia storing instructions which, when executed by one or more hardwareprocessors, cause: identifying a first call to a first applicationprogramming interface (API) point; determining that the first call tothe first API point is made in association with a first selector value;determining that the first API point depends on a parametric constant;executing a bootstrap method corresponding to the parametric constant,wherein executing the bootstrap method comprises: determining whetherthe first selector value is valid with respect to the parametricconstant; responsive to determining that the first selector value isvalid with respect to the parametric constant: returning a first anchorvalue based on the first selector value; storing the first anchor valueas a first resolved value for a first resolution state of the parametricconstant.
 2. The media of claim 1, further storing instructions whichcause: determining whether the first selector value matches any resolvedvalue for any resolution state of the parametric constant; whereinexecuting the bootstrap method is responsive to determining that thefirst selector value does not match any resolved value for anyresolution state of the parametric constant.
 3. The media of claim 1,further storing instructions which cause: determining that an operandassociated with the first call to the first API point comprises aparticular constant, wherein the first call is made from a caller type;storing, as a second resolved value for a second resolution state of theparticular constant, (a) information indicating the first API point and(b) information indicating the first anchor value.
 4. The media of claim1, wherein: the first API point is a type; the first anchor valueindicates one or more of: a first memory layout associated with aspecialized type that is the type specialized based on the first anchorvalue; a first memory size associated with the specialized type; anobject that represents the specialized type.
 5. The media of claim 1,wherein: the first API point is a field; one or more of a type and asupertype of the type defines the field; one or more of the type and thesupertype of the type specifies that an unspecialized type is associatedwith the field; the field is of a specialized type that is theunspecialized type specialized based on the first anchor value.
 6. Themedia of claim 5, wherein: the first anchor value indicates a typerestriction on the field; the type restriction prohibits a value, whichis valid for the unspecialized type, for the field.
 7. The media ofclaim 1, wherein: the first API point is a method; one or more of a typeand a supertype of the type defines the method; one or more of the typeand the supertype of the type specifies that an unspecialized type isassociated with one or more of a method parameter and a method returnvalue of the method; one or more of the method parameter and the methodreturn value is of a specialized type that is the unspecialized typespecialized based on the first anchor value.
 8. The media of claim 7,wherein: the first anchor value indicates a type restriction on one ormore of the method parameter and the method return value; the typerestriction prohibits a value, which is valid for the unspecializedtype, for one or more of the method parameter and the method returnvalue.
 9. The media of claim 1, wherein the first anchor value indicatesthe first selector value.
 10. The media of claim 1, wherein: the firstanchor value indicates a default anchor value for the parametricconstant; the default anchor value is determined during preparation of atype defining the parametric constant; the first anchor value isdetermined (a) subsequent to the preparation of the type defining theparametric constant and (b) responsive to the first call to the firstAPI point.
 11. The media of claim 1, further storing instructions whichcause: executing the first API point, responsive to the first call tothe first API point, based on the first anchor value.
 12. The media ofclaim 1, wherein determining that the first API point depends on theparametric constant comprises: determining that the first API point isassociated with a parametric attribute comprising a parametric constantreference; determining that the parametric constant reference indicatesthe parametric constant.
 13. The media of claim 1, wherein: the firstcall to the first API point comprises an instruction and an operand; theoperand is a linkage constant that is in an unresolved state; aninformation structure for the linkage constant indicates the first APIpoint and the first selector value.
 14. The media of claim 13, furtherstoring instructions comprising: resolving a symbolic reference to thefirst API point prior to resolving the linkage constant.
 15. The mediaof claim 13, further storing instructions which cause: storing a secondresolved value for a second resolution state of the linkage constantthat indicates the first anchor value.
 16. The media of claim 1, furtherstoring instructions which cause: identifying a second call to the firstAPI point, wherein the second call to the first API point is made inassociation with a second selector value different than the firstselector value; executing the bootstrap method associated with theparametric constant, wherein executing the bootstrap method comprises:determining whether the second selector value is valid with respect tothe parametric constant; responsive to determining that the secondselector value is valid with respect to the parametric constant:returning a second anchor value based on the second selector value;storing the second anchor value as a second resolved value for a secondresolution state of the parametric constant; wherein the first selectorvalue and the second selector value are different; wherein the firstanchor value and the second anchor value are different.
 17. The media ofclaim 1, further storing instructions which cause: identifying a secondcall to the first API point, wherein the second call to the first APIpoint is made in association with a second selector value different thanthe first selector value; determining that the first resolution state ofthe parametric constant, of a plurality of resolution states of theparametric constant, is further associated with the second selectorvalue; executing the first API point, responsive to the second call tothe first API point, based on the first anchor value that is the firstresolved value for the first resolution state of the parametricconstant.
 18. The media of claim 1, further storing instructions whichcause: identifying a second call to a second API point, wherein thesecond call to the second API point is made in association with thefirst selector value; determining that the second API point depends onthe parametric constant; determining that the first resolution state ofthe parametric constant is associated with the first selector value;executing the second API point, responsive to the second call to thesecond API point, based on the first anchor value that is the firstresolved value for the first resolution state of the parametricconstant.
 19. The media of claim 1, wherein the first anchor value isnot accessible by a callee type making the first call to the first APIpoint.
 20. The media of claim 1, wherein: the first API point is aclass; the first anchor value indicates an object representing aspecialized class that is the class specialized based on the firstanchor value; the object is accessible by a callee type making the firstcall to the first API point.
 21. The media of claim 1, wherein: thefirst call to the first API point is associated with an operand; theoperand is a linkage constant; no loadable value is associated with thelinkage constant.
 22. The media of claim 1, wherein: the first API pointis a class; the first call to the first API point is associated with anoperand; the operand is a linkage constant; a loadable value for thelinkage constant indicates an object representing a specialized classthat is the class specialized based on the first anchor value; theloadable value for the linkage constant is accessible by a callee typemaking the first call to the first API point.